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The Human Body Background notes 4
4. Modern maps of the mind
Until the use of modern medical imaging, the brain was really the last great, unexplored territory. The only
clues to the functions of particular areas come from injury or disease. With modern imaging technology, such
as EEC mapping, CT, MRI or PET scanning we can now follow what is happening in the healthy brain. When
a part of the brain is active the electrical activity and blood flow in that area increases. These changes can be
measured and recorded without surgical intervention.
Brain dissection
The link between body and mind continues to be explored by dissecting and examining the physical brain.
Fragments of Einstein’s brain are still being studied: does its structure reveal why its owner was a genius?
The brains of executed criminals were routinely examined last century. The brains of psychiatric patients in
Victorian mental institutions were investigated to find structural correlations with their illness. Dissection of
the brain of a gifted piano player was carried out in the hope of explaining her talent. Differences in brain size
and structure are also proposed as explanations for the differences between men and women or to explain
sexual preferences.
Brain surgery
It has been found that stimulating particular areas of the brain of a conscious patient evokes specific
memories. Operations such as these show how the brain is mapped by triggering specific structures within it.
(Left to right) : Composite MRI and PET scan illustrating the areas that are active in the brain during rest.
Source: St. Vincent's Hospital ; dissecting the brain.
Brain injury
Study of brain injuries have also yielded information about the localisation of various brain functions. Highvelocity bullets cause localised damage and loss of specific brain functions. In 1848, a railway worker called
Phineas Gage accidentally set of an explosive charge and blew a tamping iron through his head. Although
he survived the iron bar passing through the frontal lobe of his brain his personality changed so much that he
was studied extensively and his story is now to be found in most neurology textbooks. One of the most
important pieces of information gained from observations of the results of disease and injury to the living
brain was the striking asymmetry of brain function, with clear distinctions (in most individuals) between the
right and left hemispheres.
The ultimate map: the mind-body divide
Much of Western medicine has been based on a compartmentalised view of the body and mind. The body is
seen as the sum of discrete, machine-like systems, which are often treated in isolation from each other and
from the influences of the mind, and vice verse. Most of the current approaches to understanding and
mapping the human mind attempt to correlate it with the electrical, biochemical and structural events that
take place in the brain. The mind is often seen as what the brain does. Many people believe that feelings,
memories and thoughts are "no-more than the behaviour of a vast assembly of nerve cells and their
molecules’. However, much of the brain is still virtually uncharted territory.
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The Human Body Background notes 4
The major structural features of the brain and many of their functions have been identified. We know which
areas of the brain are responsible for motor control, for processing sensory input, for language. The release
of endorphins in our brain makes us feel good. Serotonin levels have been associated with depression. A
positron emission tomography (PET) scan shows increased activity in certain areas of the brain when we
think. Specific memories can be localised to specific areas of the brain and damage to the brain can
undoubtedly result in personality changes or loss of memory. However, what about personality? Where is
consciousness located? How are memories stored? Is our mind more than our physical brain? How much of
our mind influences our body?
Our thoughts and feelings are not only influenced by the electrical responses of our brain. The rest of our
body is involved as well. Fear and other emotional stresses, for example, release adrenalin from the adrenal
gland situated above the kidney, which affects many other physical responses throughout our body and
further influences our thoughts and emotions. Sexual desire also results in physical body responses.
Depression can affect muscle tone and other physical changes in the body. Likewise, physical changes in
the body can also effect changes to our thoughts and feelings. The question of the mind and body divide
continues to absorb philosophers and scientists and at the heart lies the question of their
interconnectedness.
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The Human Body Background notes 5
5. Close-ups: The microscopic world of cells
Microscopic and biochemical techniques have allowed us to look at the body in ever more detail, to go
beyond the ability of the naked eye. These techniques have revealed that each organ is made of tissues and
each tissue is made of cells. The cell, the fundamental structural and functional unit of the body, has been
shown to have its own complex structure. At an even deeper level of detail, we can look at the individual
molecules that make up and determine the structure of our cells. The techniques of dissection revealed the
structure of the body to the Renaissance world; in the late 20th century microscopic and biochemical
techniques are opening up new cellular landscapes.
Looking at tissues and cells
The decade 1660–1670 saw the opening of a new landscape for exploration. In 1661 in Bologna, Marcello
Malphigi (1628–1694) described the microscopic structure of the pulmonary capillaries. Further north in the
Dutch city of Delft, his contemporary, Antoni van Leeuwenhoek, was making tiny single double-convex
lenses mounted between brass plates and held close to the eye. These lenses magnified objects up to 300
times. The object to be viewed was mounted on the pin. One of the copper plates contained a lens, the other
a small opening through which to view the object.
Across the channel in Britain, Robert Hooke published his record of microscopic observations, Micrographia,
in 1665. Hooke was the first to describe the ‘cell’, a term derived from the Latin word for ‘small room’.
Looking at a piece of cork, he observed ‘These pores, or cells [which] consisted of a great many little
boxes… were indeed the first microscopical pores I ever saw…’.
Microscope designs progressed with the work of enthusiasts such as Edmund Culpeper in London, during
the period 1730–1740 and later in Germany, with new microscope models by Carl Zeiss in 1890. Microscope
designs continued to develop and looking through microscopes became a popular social activity in the 18th
and 19th centuries, both at meetings of learned societies open to the public, and in private homes.
Developments in microscopy and in techniques of preparing tissues and cells for examination continued to
yield ever more detailed pictures of the cellular structure of tissues, leading to a cellular inventory of the
body. Early studies were plagued by problems with optical aberrations and artefacts of staining and
preparation. Technical improvements resulted in studies of the anatomy of individual cells. Just as the body
was explored and mapped, so too was the cell. Far from being an amorphous blob, the cell was revealed to
have a complex internal structure.
In 1939, the production of the first commercial electron microscope made it possible to map the finest detail
within single cells. Electron microscopes use beams of electrons instead of light. Light microscopes can only
magnify objects by 1200 times but electron microscopes can magnify objects up to 100 000 times. It is now
even possible to use scanning electron microscopes to observe DNA molecules within the nucleus of cells.
Microscopic drawings from Hooke:
microscope and burner. Source: State
Library of Victoria
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What are cells made up of ?
No two cells are identical, but all cells have similar parts. Cells contain tiny structures, called organelles.
They are crucial for the work that a cell does and its survival. These organelles may only be seen under high
magnification. An outer skin, called the plasma membrane, surrounds a cell. Inside the cell is a thick fluid
called the cytoplasm. Suspended in the cytoplasm are organelles, which are the cell’s organs. Organelles
maintain the life of the cell.
Image sources: Molecular Probes Inc.; Medical Research Council Laboratory of Molecular Biology; Brigham and Womens's Hospital.
Living things - one cell or many
Single cell organisms
Some microorganisms, such as bacteria, exist as a single, prokaryotic cell.
This means that they do not have a nucleus and other membrane-bound
organelles. They have a single, relatively small, circular strand of genetic
material that determines their non-complex lifecycle. One cell carries out all
of the operations that are needed to survive – biochemical reactions known
as metabolism, growth and reproduction.
E.coli - bacterial cell. Source:
AMGEN Australia
Multi-cell organisms
Other living things develop into more complex multi-cellular organisms, such as fungi, plants and animals.
Because cells work best at a large surface area to volume ratio their ideal size is quite small. Organisms
have overcome this size limitation and grown out of the microscopic world by increasing the number of cells
that they contain. Individual cells within an organism have become specialised to carry out specific functions.
These organisms contain eukaryotic cells that have a membrane-bound nucleus with abundant genetic
coding material and an assortment of different organelles, which perform a range of functions inside the cell.
The genetic material inside most eukaryotic cells occurs in pairs, known as homologous pairs – one of each
pair contributed by each of the parents.
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What does a cell do?
All cells carry out the essential housekeeping of living things. Our cells behave like us. They eat – they
absorb sugars and other nutrients across their membrane. They move – some swim, some crawl and others
bend and flex. Cells have a flexible skeleton called a cytoskeleton that acts as the cell’s muscles and bones.
They can defend themselves from attack by viruses and bacteria and they also reproduce themselves. Cells
have a lifespan that determines when they become old and die. Major responsibilities carried out by all cells
include chemical reactions or metabolism, growth, and reproduction.
Metabolism
Metabolism describes the complete range of chemical reactions and biochemical substances that occur in a
cell, involved in the building up and breaking down of biological chemicals and food. This process is linked to
the production and management of heat and chemical energy for the cell and for the entire organism.
Reproduction
Reproduction is a fundamental characteristic of cells and all living things. It ensures the continuation and
survival of the species after the death of the individual.
Asexual reproduction – mitosis
Most single cell (prokaryotic) organisms reproduce themselves by a process called mitosis, where a parent
cell divides into two identical daughter cells. In turn these cells divide and produce two more identical cells,
and so on.
(Left to right) : Cell in telophase phase of mitosis.. Electronmicrograph of cell division –mitosis. Source: National University Hospital of
Singapore.
The new cells have exactly the same genetic material as the original cells. They are clones. When an
organism produces offspring in this manner, without any change or exchange of genetic material, it is
referred to as asexual reproduction. Bacteria and moulds can undergo asexual reproduction and produce
colonies of millions of genetically identical cells from one original cell, within 24 hours.
How do these clonal organisms maintain genetic diversity, which is so
important for the adaptation and evolution of organisms? Random errors
that occur in the genetic material of microorganisms – that reproduce
rapidly creating colonies of millions of cells – are sufficient to continually
provide new genetic variations and diversity within the species.
Bacteria can also exchange useful genetic material between themselves
on small, circular stands of DNA, called plasmids. It is this process that
has allowed them to quickly evolve resistance to antibiotics since their
application began in the 1940s.
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Some multicellular organisms are also able to reproduce asexually. They do this by separating cells or tissue
from the parent, which then divide by mitosis and grow into a complete new organism. New organisms
produced in this way are also clones of the parent. Many plants such as potatoes and some animals such as
starfish will reproduce in this way if the circumstances call for it. However, in animals this is usually an
aberration of the normal breeding cycle.
Sexual reproduction – meiosis
Large multicellular (eukaryotic) organisms such as humans that reproduce more slowly than microorganisms
have evolved another efficient way of exchanging genetic material and ensuring diversity in our populations.
Sexual reproduction, involves exchanging and reshuffling genetic material from two parents instead of one,
to produce unique genetic combinations and unique individuals. This process requires special sex cells,
called gametes, which are produced in the reproductive organs. Gametes in human males are called sperm
and in human females eggs or ova
Meiosis - making sex cells for sexual reproduction
Most cells in the body are called somatic cells and contain two
copies of the genetic code. Each copy is inherited from one of
the parents. Gametes, however, contain a single copy of the
genetic code, which explains why they are often referred to as
half cells.
Gametes are produced in sexual organs by a special type of
cell division called meiosis. During meiosis the cell duplicates
each pair of chromosomes, shuffles the genes within the
chromosomes and divides twice. This process results in four
gametes. Each of these gametes contains a single unique
arrangement of the genetic code
Fertilisation
During fertilisation a male and female gamete fuse to form a
new cell called a zygote. The arrangement of genetic material
inside this first cell is reflected in every cell of the new
organism.
The gametes, which came from each of the parents,
contained an assortment of genetic material which in turn has
come from each of the grandparents and so on. Genetic
information is exchanged in this way through family lines,
within populations, and within a species, from generation to
generation.
Process of meiosis
Process of fertilisation over two generations –
grandparents fertilise parents which in turn
fertilise offspring.
Sperm surrounding an egg cell. Source: National
University Hospital of Singapore
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Cell growth and specialisation
Over the gestation period, an individual develops the tissues, organs and systems to survive after birth. From
the moment a zygote is formed growth occurs by mitosis. Each new cell is produced by the division of a preexisting cell. The zygote divides by mitosis through several stages, first into an embryo and then into a fetus.
After the first week of division a cluster of cells exists, called a blastocyst, which contain inside it early nonspecialised cells called embryonic stem (ES) cells.
ES cells have the potential to become any cell of the body – blood, nerves, muscle etc. At some point these
cells encounter chemical triggers that cause them to start to differentiate into specialist cells. This means that
they start to read different parts of the genetic code and perform different duties, they even change their
shape to suit the task they will perform for the rest of their life. Once differentiation happens, cells are able to
do certain things very well, but lose their ability to do other things.
In this way an embryo develops into a fetus and into a unique individual with fully developed tissue, organs
and functioning body systems. This individual continues to grow into an adult, which is able to reproduce
itself. Only at full growth will the rapid division of cells slow down. Mitosis continues at a much slower rate,
producing new cells only to replace those that are damaged or have died.
Cell… tissue… organ… system
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Humans have 216 different cell types throughout our body and approximately 10 or a hundred thousand
billion cells in total. Most cells are very, very small – a single drop of blood contains over 200 million red
blood cells. Cells are so small that we cannot see them unless they are magnified.
Multi-cellular organisms, such as humans, display extraordinary organisation and cooperation between the
different cells, tissues and organs that carry out the many tasks that sustain life. A collection of similar cells
that work together to carry out a specific function is called tissue. Organs are made up of a number of
different tissues that work together to carry out a more complex function for the body. A body system is a
term that refers to the action of a number of organs and tissues that work together to carry out more
specialised functions within the body.
For example, the digestive system is made up of many different organs including the stomach, intestines,
pancreas and liver. The organ known as the liver is made up of bile ducts, blood vessels, blood and liver
tissue. Liver tissue is made up of liver cells, which are highly specialised to transform or breakdown
dangerous chemicals from the blood, and produce other substances that assist with the body’s waste
elimination.
(Left to right) : Embryonic stem cells grown to produce specialised cells; Embryonic blastocyst containing ES cells Source: Monash
Medical Centre.
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The Human Body Background notes 5
Whatever its function, a tissue's cells are right for the job. Fat cells are round, with a large internal volume,
enabling them to store fat droplets. Skin cells are flat and fit together to form a protective barrier. Red blood
cells are flexible and can squeeze through very tiny vessels. Nerve cells have long extensions that make
contact with a variety of other cells, enabling them to carry messages from one part of the body to another.
Sperm have tails, which propels them towards the egg. Secretory cells have large capsules inside that
contain hormones or enzymes.
nerve cells
skin cells
intestinal cells
white blood cells - macrophage
When things go wrong
German pathologist Rudolf Virchow (1821–1902) described the body as a ‘cell state… in which every cell is
a citizen’. He described disease as a state where these citizens are in conflict.
As with studies of gross anatomy, the aim of looking in detail was not only to see what cells looked like and
to speculate how they worked, but to establish a basis for diagnosis, to set the boundaries of health and
normality. Cancer cells look different from healthy cells; the red blood cells from a patient suffering from
Sickle Cell Anaemia have a distinctive shape. Just as medical images are used for screening, so too is the
examination of our tissues and cells a basis for screening. For example, pap smears are used to diagnose
cervical cancer and colon biopsies are used to diagnose colon cancer.
Even examination of the detailed infrastructure of the cell is used to screen for or diagnose defects.
Individuals suffering from Down’s syndrome (also known as trisomy 21) have three copies of chromosome
21 in place of the normal two.
Variation in cells and tissue can be observed with high powered microscopes. The changes in the cells and
tissue often indicate different diseased states. For example, microscopy of sperm cells can often reveal
reasons for infertility.
(Left to right) : Acrosome-intact sperm; Sperm with a defective nucleus; Round-headed sperm with no acrosome - Source: National
University Hospital of Singapore
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Many early studies involved killed tissues and cells. These were treated with various stains to emphasis
different features. Improvements in microscopy such as the phase contrast microscope and the laser
confocal scanning microscope mean that living cells can now be viewed in action, and even modified in
surgery – removing or adding various organelles – the ultimate in microsurgery.
The techniques of histology (study of tissues) and cytology (study of the cell) have revealed the structure of
the body and the cell in minute detail. Such details provide clues to the mechanisms of physiological
processes. The receptors and channels of the cell membrane, for example, help us to understand the way
hormones, neurotransmitters and certain drugs work. These approaches are now also used to tackle the
problem of mapping the brain. By tracing the connections between nerve cells, will we learn how we are able
to learn? Will the maze of cellular networks or the internal structure of the cell provide a map with clues to the
nature of consciousness?
What if the plasma membrane didn’t work properly?
Sometimes genetic errors or environmental events cause things to go wrong with some of the organelles in
cells. Cystic fibrosis is a genetic condition that affects molecular pumps that regulate the ionic concentrations
across the plasma membrane does not function properly. Certain minerals, and consequently water, are
unable to pass through the membrane and into the cell. One of the symptoms of this condition is the
production of thick mucus that collects min the lungs.
What if lysosomes couldn’t become acidic?
Sometimes invasive organisms can also affect the organelles of cells. If someone has tuberculosis, the acid
content of lysosomes can be neutralised by the bacteria that cause this disease. The lysosomes are
rendered useless and fail to digest the bacteria causing them to thrive within the cell.
What if mitochondria couldn’t produce energy?
Sometimes environmental factors can adversely affect specific organelles in cells. Cyanide is a poison that
can kill a person because it blocks the important energy-producing chemical reactions that occur within the
mitochondria. Without energy the brain, heart, and every cell of the body quickly stops functioning and dies.
Can a cell keep dividing?
Cancer is a very common disorder, caused by a range of environmental or genetic factors that affect the
genetic material in the nucleus of cells. The changes in the genetic material affect one or more proteins that
are involved in regulating the life cycle of a cell. This results in the uncontrolled division of cells, which collect
into a tumorous mass. Cancer may also occur when cells do not die when they should.
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The Human Body Background notes 6
6. Close-ups: The microscopic world of DNA
Looking at molecules
The microscope extended our vision and allowed us to recognise the cell as the basic unit of life. The
electron microscope revealed the structure of the cell. To determine the structure of molecules, the
chemicals that form the building blocks of the cell, we turned to chemistry and physics. Today’s molecular
biologists are continuing the anatomical tradition, opening up and charting new landscapes currently beyond
the reach of human vision. Biochemical techniques reveal the molecular composition of the various parts of
the cell. Two classes of molecules, proteins and nucleic acids (DNA), show great variability in their
structures. These structures can also be mapped to the functions that they perform within the body to extend
our current notions of normality and health.
DNA - The Universal Code
Deoxyribonucleic acid (DNA) is the common genetic material of all
living things. It is found in all cells from bacteria to humans and
contains the instructions to make and regulate the entire organism.
DNA carries information for the manufacture of proteins. It is also the
molecule transmitted from one cell division to the next, from one
generation to the next and as such, is the basis of heredity.
DNA is made-up of two long strands that wind around each other, like
a twisted rope-ladder. This structure is called a double helix. Each
strand of DNA is like a string of beads, where each bead consists of a
phosphate unit, a sugar (deoxyribose) unit and one of four different
base units, called nucleotides – adenine (A), cytosine (C), guanine
(G), and thymine (T). The order of the nucleotides (bases) within the
DNA strand determines the instructions coded within it.
A sequence of bases that is the code for a particular protein is called a
gene. As with proteins, a change in even one of these bases can
change the function of the gene or result in a gene that makes
proteins that no longer functions. Defective genes, associated with
many diseases have now been located and identified, opening the
way for screening of such diseases as cystic fibrosis, muscular
dystrophy and Huntington’s chorea.
A section of DNA double strand,
showing the complementary base
pairing arrangement. Source: Janssen Cilag Pty Ltd
What are chromosomes?
In the cell, DNA is arranged into small structures called chromosomes.
These can be seen in cells that are about to divide, using highpowered microscopes.
There are 46 chromosomes in every human cell. One complete set of
23 chromosomes is inherited from the female parent and another set
of 23 is inherited from the male parent. The chromosomes of higher
organisms are made of tightly packed strands of DNA coiled around
balls of protein, called histones.
DNA is so tightly coiled in chromosomes that if it was unravelled from
one human cell, it would stretch almost two metres long. If the DNA
from every cell of one person was joined end to end it would stretch to
the moon and back – over 700,000 km.
Chromosomes are made of DNA coiled
around proteins called histones. DNA
strands are coiled together into a
double strand called a double helix.
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The Human Body Background notes 6
What are genes?
Genes are discrete segments of DNA. The DNA sequence in genes tells cells exactly which proteins to make
and the order of the amino acids in the protein sequence. Our genes carry the DNA instructions to make
every protein in the body. How an individual grows, develops and functions is determined by these protein
products. It has recently been estimated that human beings have about 30 thousand genes in our DNA. One
gene may actually code for several slightly different proteins by exchanging short sections of its DNA
sequence when it is read.
Although the number, and type, of genes in a species are fairly constant, no two people – with the exception
of some twins – have exactly the same combination of genes in their cells. Individuals develop their own
unique characteristics. Variations of a functioning gene are called alleles. A gene for eye colour may have
several different alleles – each with minor changes to the nucleotide sequence in the gene – which produce
slightly different proteins, resulting in brown or blue pigment. Despite these variations most of our DNA is
identical to the DNA of everybody else. The way our bodies develop, what we are able to do, and how we do
it is similar within species. In fact, different species also share many common genes. For example, the DNA
sequence of a chimpanzee is almost 98.5 % the same as humans.
What is protein?
Proteins are complex molecules made by the cell. They are the worker and structural molecules that allow
living things to function properly. Proteins are made up of individual units, called amino acids. There are 20
different types of amino acids with different properties (size, ionic charge or pH) that affect the way the final
protein folds and behaves when it is made. Proteins contribute to our physical and chemical characteristics.
Different proteins do different things. Hormones such as insulin or growth hormone are examples of
messenger proteins, secreted from cells to travel to other tissue in the body and trigger specific responses in
them. Proteins, such as digestive or pancreatic enzymes, catalyse metabolic reactions, and structural
proteins, like those found in the cytoskeleton of cells or the contractile fibres in muscle cells, allow the body
to move and support itself. Some proteins, such as receptors or immune proteins (antibodies), change their
shape and trigger reactions when they bind to other proteins or strange chemicals and cells, (for example
bacteria or chemical toxins).
Even a minor change in the amino acid sequence can result in a major change in how the protein functions.
Haemoglobin, for example, is a protein consisting of two alpha-chains and two-beta chains, each around 140
amino acids long. Sickle-cell anaemia results from the substitution of just one amino acid in the beta chain.
Determination of the three-dimensional structure of haemoglobin, the red pigment in blood, provided the final
piece in the explanation of the circulatory system, a story that had been begun by William Harvey in 1628.
The haemoglobin molecule can reversibly bind oxygen, picking up oxygen in the lungs and releasing it at
other sites in the body. The distortion in structure found in the sickle-cell haemoglobin interferes with its
ability to bind oxygen and explains the clinical features of the disease.
What is the DNA code?
How does a cell know the sequence of amino acids that go together
to make a protein? In the language of DNA, a sequence of three
nucleotides is called a triplet, or a codon. Codons are read by the
cell like words. The cell reads the codons and translates them into
amino acids in a newly constructed proteins sequence. For example,
the nucleotide, sequence ATCTTACCA, is read by the cell as three
triplets, or ‘words’, ATC, TTA, and CCA. The cell translates the
‘words’ into an amino acid sequence. The amino acids isoleucine,
leucine and proline are joined together, in that order.
With only four nucleotide letters in the DNA alphabet (A,T,C,G) the
DNA language consists of only 64 different codons, or ‘words’
(4x4x4 =64). However, there are only 20 amino acids that the cell
can use when it is making a new protein. Most of the amino acids
are coded by several different codons. In this DNA language, there
is also a special triplet (ATG) that signals the first amino acid (Met)
of the protein sequence. There are several other triplets (TAA, TAG,
and TGA) that signal the end.
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The Genetic Code.
Source: Janssen – Cilag Pty Ltd
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The Human Body Background notes 6
DNA copies itself
After a cell divides, the two new cells each have the same DNA. This is
because DNA can make an exact copy of itself. First, the two strands of
the double helix become unzipped with the help of an enzyme, called
DNA polymerase. This protein moves along the single strands and
builds a complementary second strand of DNA from free-floating
nucleotides. The cell then has two copies of its DNA and is ready to
divide. This process is called DNA replication.
In short, living things are made up of cells. Cells contain DNA, which
consists of genes. Genes make proteins. Proteins in turn make cells
and help them replicate, which make up living things.
When things go wrong
DNA doesn't always copy itself perfectly. Mistakes in the genetic code
are called mutations. We all carry mutations in our DNA. A point
mutation occurs when there is an error in one nucleotide unit. For
example guanine (G) nucleotide may be substituted by adenine (A).
This means that the message to make a specific protein is altered and
the cell can no longer produce it in exactly the same way. A deletion is a
type of mutation where a segment of the chromosome, or a long section
of DNA, is not copied and is missing from the replicated strand of DNA.
Depending on how large the deletion is, the protein is either incomplete,
incorrect or not made by the cell. A translocation is a type of mutation
where large sections of the code are inserted in the wrong place. This
means that the information is scrambled from the point of the
translocation and proteins that are encoded in this region are either
incorrect or not made by the cell.
DNA replication occurs when two
strands of the double helix unwind and
each specifies a new daughter strand
by base-pairing rules. Source: Janssen
– Cilag Pty Ltd
Many changes or mutations occur during a lifetime, when the DNA in cells copies itself incorrectly. If a
mutation occurs in the sex cells of an individual it may be passed on from generation to generation.
Sometimes a mutation will cause a noticeable change to the way an organism appears or behaves.
Sometimes the change will have no effect. Occasionally mutations may result in favourable characteristics.
Some mutations make only small changes to the way the body functions or appears. Perhaps they determine
different abilities or different physical characteristics, such as whether you have dimples, freckles, a widow’s
peak or a straight hairline. Simple mutations can determine the shape of your ears and even the shape of
your thumbs.
These photographs show the difference between a widow’s peak hairline and a straight hairline.
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The Human Body Background notes 6
Mutations in DNA can sometimes cause problems to the way the body
functions. The change may mean that someone produces a defective
protein, or too much or too little of a protein. Just like other characteristics,
these changes can be inherited. Down’s Syndrome is an inherited genetic
condition that causes intellectual disabilities due to a damaged or additional
chromosome.
Some mutations remain in a population because they seem benefit the
population as a whole − even though individuals may develop the disease
symptoms. For example, the mutation that causes sickle cell anaemia can
protect carriers from malaria. Those with a single mutant gene will benefit
from its anti-malaria effects but those carrying two mutant genes experience
the disease symptoms of this type of anaemia.
Mapping DNA and chromosomes
You can see DNA through the electron microscope but it is impossible to see
its detailed structure. Strands of DNA can be mapped against each other
using biotechnological tools and processes such as gel electrophoresis or
PCR techniques. Analysing DNA fragments reveals information about the
individual.
DNA fingerprint
Source: Victoria Police, Victoria
Forensic Science Centre
Chromosomes can be identified with a light microscope by their size and shape. The number and type of
chromosomes reveals different information about your identity. There are 46 chromosomes in most human
cells. Females have two X chromosomes. Males have one X chromosome and one Y chromosome.
Chromosomes can also be mapped to identify the location of particular genes. These maps are made by
analysing the patterns of inheritance in families.
(Left to right) : Female chromosome pattern and male chromosome pattern. Source: Royal Children's Hospital. To make these pictures,
the nuclei of cells are photographed. The chromosomes are individually identified, cut out of the photograph and lined up against their
pair, in the following arrangement. This family tree shows how patterns of inheritance are passed down in families. Source: The
University of Melbourne.
The last maps
On 1 October 1989, the Human Genome Project, one of the most ambitious cartographic exercises on
humans, began. This project aimed to determine the detailed DNA sequence of the complete human
genome. In parallel with this effort, the DNA of a set of model organisms is also being studied to provide
comparative information necessary for understanding the functioning of the human genome.
In April 2003, those scientists involved in the Human Genome Project completed the task of sequencing the
approximate 3 billion pairs of DNA found in the humans. Since then, efforts have been focused on
determining the location and function of an estimated 100,000 human genes. The information generated by
the human genome project will be of immense benefit to biomedical science and may eventually help us
understand and treat many of the 4000 or more genetic diseases that afflict humankind and the multifactorial
diseases in which genetic predisposition plays an important role.
http://museumvictoria.com.au/education/
A Museum Victoria experience.
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