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Chapter 16 DNA & RNA
Heredity (Genetic Inheritance)
Heredity is the passing on of features from parents by
means of genes.
Humans inherit features such as number of fingers.
Plants inherit features such as number of petals.
Genes
A gene is a section of DNA that causes the production of a
protein.
Genes control a cells activities.
Genes are the units of heredity.
Gene Expression
Gene expression is the way in which the genetic information in a gene
is decoded in the cell and used to make a protein (Gene expression
refers to the way genes work).
Characteristics are traits or features that are inherited genetically,
characteristics arise from the interaction of heredity and your
environment.
A child may inherit the gene for
tallness but if the child’s diet lacks
the correct nutrients the genes may
not be able to cause tallness i.e. the
genes may not be expressed.
Chromosomes
Chromosomes are composed of 60%
protein and 40% DNA
• DNA –
Deoxyribonucleic Acid
• The protein in DNA is
responsible for holding
the DNA in a tightly
packed configuration so
it can fit into the
nucleus.
• DNA is heavily folded
and coiled to fit into the
nucleus.
• A typical human
chromosome had a
DNA strand 6cm long
Non-coding DNA
Non-coding DNA (junk DNA) is
DNA that does not carry the code
for the formation of a protein.
a)
b)
Recent research shows that large
amounts of this non-coding DNA
may act as a genetic control panel
switching genes on and off.
Two types of non-coding DNA
Some of it occurs between genes
Some of it is found within genes
Structure of DNA
4 Chemicals called bases are used in DNA
adenine (A)
thymine (T)
guanine (G)
cytosine (C)
Each of the 4 bases can only join or
bond with one other base.
A joins with T
G joins with C
The pairs A/T and G/C are called
complementary base pairs
Remember:
At The Giant’s Causeway
The DNA Molecule is made of two
attach strands similar to the two sides
of a ladder.
The strands are held together by
complementary bases.
If one strand of DNA has the sequence
TAGCAT then the sequence on the
partner strand must be ATCGTA
DNA is arranged to form a double helix
shape
Genetic Code
The genetic code is the sequence of bases in DNA that
provide the instruction for a cell (Using RNA) to form a
protein.
Human chromosome Number 1 has about 300 million base
pairs. The precise sequence of bases is called the genetic
code.
Each group of three bases (triplet
or Codon is the code for an amino
acid.
A triplet (codon) is a sequence of
3 bases in DNA (or RNA) that
codes for an amino acid
A sequence of bases that
produce a protein is called a
gene.
DNA (or a gene) makes protein as follows:
• the DNA strands separate
• the bases on DNA link up with complementary bases to
form mRNA (the code is transcribed)
The detailed structure of
DNA consists of:
• nucleotides, which contain
phosphate, deoxyribose sugar
and a base. The four bases are:
the purines adenine (A) and guanine (G)
the pyridimines thymine (T) and cytosine (C)
• nucleotide or base pairs, i.e. A=T or
G≡C, join together due to hydrogen
bonding
•The base pairs are held together by
hydrogen bonds.
• a double helix, where the
phosphates and sugars form the sides
of the molecule and the base pairs are
like rungs inside the double helix
DNA Replication
At the end of mitosis each new cell has a single
stranded chromosome. Before the cell can divide
again the DNA strand must produce a new copy of
its self. This process is called DNA Replication .
This takes place in the nucleus during interphase.
Mechanism of DNA Replication
1. The double helix unwinds.
2. An enzyme breaks the bonds between the base
pairs.
3. DNA bases present in the cytoplasm enter the
nucleus. The incoming bases attach to the exposed
Complementary bases (base pairing occurs)
4. Each side of the DNA molecule acts as a mould
for the new DNA strand that is formed.
5. Each new double strand rewinds to form a
double helix.
Each new DNA double helix is:
Half new DNA and half original DNA
Identical to the original DNA double helix and to
the other new
double helix.
DNA Replication
Significance of DNA Replication
Each new strand of DNA will have exactly the same sequence of
bases as the original DNA strand.
This allows the same DNA to be passed on to each new
generation of cells.
DNA Profiling
A DNA Profile (DNA or genetic
fingerprint) is a method of making
a unique patter of bands from the
DNA of a person, which can then
be used to compare with the DNA
profile of another person.
Method of Preparing a DNA
profile
Preparing a DNA Profile
Involves 4 steps:
1. Releasing DNA from cells
2.Cutting the DNA into
fragments using restriction
enzymes
3. Separating the fragments
according to their size
4. Comparing the patterns of
the bands
DNA profiling in more detail
1. Cells are broken down to release DNA
If the amount of DNA is too small to work with it can
be increased (amplified) using the polymerase
chain reaction (PCR).
2. DNA is cut into fragments
Isolated DNA is cut into fragments using special
enzymes called restriction enzymes. Different
restriction enzymes cut the DNA at specific base
sequences. e.g. one restriction enzyme always
cuts the DNA at the sequence CAATTC
3. The fragments are separated
The section of DNA that have been cut are separated on
the basis of their size using a process called gel
electrophoresis. An electric current is applied along a gel
in a small glass tank. The current draws the negatively
charged DNA to one end of the gel. Small DNA
fragments move faster along the gel than larger DNA
fragments.
When the electrophoresis is finished a permanent record
is obtained by adding radioactive material which
combines with the DNA fragments. A photographic copy
of the bands is then obtained.
4. Patterns are compared
It is highly unlikely that two people will have the same DNA
Profile (unless they are identical twins).
Application of DNA Profiles
DNA profiles can be used to:
• Forensic Science. To establish whether
biological tissue e.g blood, hair, saliva or semen at
a crime scene matches or does not match a
suspect.
(Forensic Medicine is the way in which medical
knowledge is used in legal situations to convict
criminals)
• Medical. To determine whether a person is or is
not the parent of a child
Genetic or DNA screening means that a person’s
DNA can be tested to show the presence of
normal or altered genes (which may cause
disease).
To decide if a man is the father of a child
blood samples are taken from the child,
the mother and the man. DNA profiles are
then prepared and examined.
If all the bands in the child’s profile math
with the bands in either the mothers or the
mans profile then the man is shown to be
the natural father of the child.
If some of the child’s bands match those of
the mother but the rest do not match those
of the father then the man is not the
natural father of the child
Genetic Screening
Genetic screening means testing DNA for the presence or
absence of a particular gene or altered gene.
Genetic Disorders caused by defective genes include:
a) Albinism (pigment melanin cannot be made in skin)
b) Cystic fibrosis (build up of mucus in the lungs and
intestines)
c) Haemochromatosis (in which too much iron
accumulates in the body and has to be removed by
regular bleeding)
d) Some cancers
Genetic screening can be carried out in two main ways
1. Adult screening
2. Foetal screening
1. Adult Screening
Sometimes carried out on adults who although
they do not suffer from a genetic disorder
themselves may carry a defective gene in their
cells (carriers). It is now possible to identify
“carriers” of many disorders e.g. cystic fibrosis,
sickle cell anaemia etc.
People who are carriers are may be given
information about the chances of them having
a child with the disorder which allows them to
prepare for a medical condition that might
affect their family.
2. Embryonic or Foetal Screening
Cells are removed from the embryo, placenta or the fluid around the
foetus. These cells can be tested to detect if the embryo or foetus
has one of a number of genetic disorders.
Ethics of Genetic screening
Ethics relates to whether behaviour is proper or improper.
Genetic screening may cause ethical problems
If the results are released to the public people involved may
feel embarrassed, be treated unfairly, be isolated, lose
their jobs etc.
If genetic screening is carried out on a foetus it may
encourage termination of the pregnancy?
RNA (ribonucleic Acid)
Also a nucleic acid
Consists of 4 bases: adenine, guanine, cytosine
and uracil
A and U are complementary bases and G and C are
complementary bases
RNA is single stranded
The sequence of bases in RNA depends on the
sequence of bases on the DNA strand.
e.g. If DNA strand has bases GGAATC then RNA will
have the complementary sequence CCUUAG
DNA is always found inside the nucleus but RNA
can move out of the nucleus into the cytoplasm
Summary table: Differences between RNA & DNA
DNA
RNA
Structural differences in DNA & RNA
DNA
RNA
Contains deoxyribose
Contains ribose
The bases are ATGC
(thymine)
The bases are AUGC
(uracil)
Double stranded
Single stranded
CAT GUT
New Vocabulary
mRNA is messenger RNA which is RNA which
can leave the nucleus and travel into the
cytoplasm
rRNA is ribosomal RNA which is used to make
ribosomes (organelles in the cell).
tRNA is transfer RNA is a type of RNA involved
in making proteins in a ribosome
New Vocabulary
Transcription is the copying of a sequence of genetic bases
from DNA onto messenger RNA (mRNA)
Translation is the conversion of a sequence of genetic
bases on messenger RNA into a sequence of amino acids.
Steps in protein synthesis
(ordinary level):
1. Enzymes open up the DNA at the site
of a gene
2. The DNA code is transcribed (copied)
onto a complementary mRNA strand
3. mRNA enters a ribosome in the
cytoplasm
4. Every tRNA has a complementary triplet to the triplets on
the mRNA
5. tRNA molecules enter the ribosome
6. every tRNA has a specific amino acid
7. The amino acids are attached to
each other at the ribosome to form a protein
8. The protein folds into shape
Detailed structure of DNA
The structure of DNA was worked out by
James Watson and Francis Crick.
They shared the Nobel Prize in 1962
DNA is made up of units called
nucleotides. These are arranged in very
long chains called polynucleotides.
Nucleotides
Nucleotides consist of 3 parts:
A phosphate group, a sugar and a nitrogen
containing base.
The sugar deoxyribose is a 5
carbon sugar similar to ribose
The phosphate group PO4 is
normally written a P
There are four distinct bases
i.e. A,T,G,C
Purines & Pyrimidines
Of the four nitrogenous bases two are classified as
purines (adenine and guanine) and the
pyrimidine (thymine and cytosine)
Purine bases can be remembered by the phrase:
The Attorney General is Pure
The pyramidines can be remembered because:
The pyrimidines contains the letter “Y” i.e.
thymine and cytosine
Double Helix
Protein Synthesis Extended Study
Initiation: Starting the process
1. The DNA double helix unwinds at the site of the gene
that is going to produce a protein
Transcription: Rewriting the code from DNA to RNA
2. RNA bases which are present in the cytoplasm move
across the nuclear membrane and match up with
complementary bases on the DNA strand
3.
RNA polymerase (enzyme) causes the sequence of RNA
bases to join together to form mRNA. A sequence of
three bases on DNA or RNA is called a codon.
Codons may cause three possible outcomes
a) A start codon indicates the beginning of a gene
b) Most codons in a gene specify for a particular amino
acid
c) A stop codon indicates the end of a gene
4. Every gene has one start codon, many codons specifying amino
acids and one stop codon.
Translation: the production of a protein according to the RNA code
5. mRNA moves from the nucleus into the cytoplasm
6. Ribosomes are made up of rRNA (ribosomal RNA) and proteins.
7. The mRNA strand forms weak bonds with the rRNA in a ribosome.
This will be the site of protein synthesis
8. The cytoplasm contains a supply for tRNA (transfer RNA). Each
tRNA carries:
a)
A specific triplet or anticodon
b)
A particular amino acid which is specific to the anticodon.
9. tRNA molecules are attached to the mRNA that is in the
ribosome. Each anticodon on a tRNA is complementary to
a codon on t he mRNA. The tRNA molecule enters the
ribosome
10. The first tRNA moleule will be attracted to the mRNa ust
after the start codon.
11. In the ribosome amino acids are detached form the tRNA
molecule and are bonded together to form part of a new
protein
12. tRNA moleules leave the ribosome without any attached amino
acids. As they leave they pull the mRNA strand through the
ribosome.
13. tRNA molecules continue to bind with mRNA until a stop codon is
reached.
At this point:
a)
The mRNA code sequence is complete
b)
The new protein is produced.
Functions of the three types of RNA
Type of RNA
mRNA (messenger RNA)
tRNA (transfer RNA)
rRNA (ribosomal RNA)
Functions
Type of RNA
Functions
mRNA (messenger RNA)
Complementary strand to DNA
Carries instruction for the
production of a protein from DNA
to a ribosome
tRNA (transfer RNA)
Has a complementary anticodon
to mRNA codon
Carries an amino acid to the
ribosome
rRNA (ribosomal RNA)
Forms part of the structure of a
ribosome
Forms a weak bond with mRNA
in the ribosome