Download Chapter-9-Chromosomes-and-DNA-Replication

Higher Human Biology
Unit1: Cell function and Inheritance
Chapter 9: Chromosomes
and DNA Replication
Learning Intentions.
• Describe chromosomes as vehicles of
inheritance.
• Specifically explain about....
1. Genes as regions of chromosomal DNA.
2. DNA replication, its importance, and its
relationship to nuclear division.
3. The normal chromosome complement,
homologous pairs, autosomes and sex
chromosomes.
DNA
• DNA is often
called the
blueprint of life.
• In simple terms,
DNA contains the
instructions for
making proteins
within the cell.
Why do we study DNA?
We study DNA for many
reasons, e.g.
• Its central importance
to all life on Earth.
• Medical benefits such
as cures for diseases.
• Better food crops.
Maurice Wilkins (1916-2004) and
Rosalind Franklin (1920-1958)
 Used X ray
crystallography to
determine chemical
structure of DNA
 Research formed
bases of Watson and
Crick’s DNA double
helix theory
X-Ray diffraction of DNA
James Watson(1928-)
and Francis Crick (1916-2004)
Determined
double-helix
structure of DNA
in 1953.
DNA Structure
Sugar-phosphate
backbone
Base
DNA Structure – the instructions
to make you is in your genesy
Genes and Chromosomes DNA
Structure
• A gene is a unit that
controls inherited
characteristics.
• Genes are held on
chromosomes.
• There are 46
chromosomes in the
each human body cell.
These are arranged as
23 pairs.
• In sex cells (Egg and
sperm) there are only
23 chromosomes.
Chromosome
made of
hundreds of
genes
Centromere
not made of
genes
Site (locus)
of a gene
Chromosome Complement
• Each DNA strand is split into chromosomes
• All sexually reproducing animals have pairs of
chromosomes.
• One set of each pair comes from each parent species.
• Each species of plant or animal has a characteristic
number of chromosomes (chromosome complement).
–
–
–
–
–
Humans have 23 pairs
Lions have 19 pairs
African wild dogs have 39 pairs
Mosquito have 3 pairs
Some ferns have 500-600 pairs
Autosomes and Sex Chromosomes
• In the human chromosome complement there
are 22 pairs of homologous chromosomes
called autosomes and play no part in sex
determination.
• The last pair- pair number 23- determine the sex
of the individual and are known as the Sex
Chromosomes.
autosomes
Sex
chromosomes
Human Karyotype
• 22 pairs of homologous
chromosomes –
autosomes.
• 1 pair – sex
chromosomes.
• All cells have double set
of chromosomes
(diploid) except gametes
which have 1 set
(haploid).
Human Male Karyotype
Remember the karyotype looks
different for males and females
Diploid and Haploid
• A haploid cell (e.g. a sex cell)
has a single set of
chromosomes (one of each
type).
• A diploid cell: has a double set
of chromosomes (two of each
type which form pairs).
• The chromosome complement
of a haploid cell is represented
by the letter n.
• The diploid by 2n.
Diploid/Haploid numbers in various
species
WHAT’s THE POINT CALLER!!!!!! The diploid
number is always double the haploid number!
The Human Life Cycle
• From the time of conception our cells carry
out mitosis.This allows us to make more
and more cells and become bigger. Our
cells also carry out mitosis if we need to
repair damaged cells.
Human sperm fertilising
an egg. The fertilised eg
will now be known as a
zygote.
Fertilisation
A baby starts life as a zygote (a fertilised egg). The zygote is one cell containing 46
chromosomes - 23 from the sperm and 23 from the egg. The zygote undergoes
mitosis to eventually become a fully grown baby made of many cells. It continues
to carry out mitosis as it grows and repairs throughout life.
There are two types of Nuclear
Division
• Mitosis – Nucleus of normal body
cell divides into 2 daughter nuclei
• Meiosis – Sex cell production.
Nucleus of gamete mother cell
divides into 4 daughter nuclei with
23 chromosomes.
Mitosis – We will expand on
mitosis and meiosis in chapter 10
• Mitosis is the process by which the nucleus of a
normal body cell (or zygote) divides into 2
daughter nuclei.
• Each of these receives exactly the same number
of chromosomes as were present in the original
nucleus.
• Mitosis is followed by division of the cytoplasm to
form two daughter cells and sometimes the entire
process of cell division is loosely referred to as
MITOSIS.
This is how the body makes new cells
when growing or repairing damaged cells
Spindle
fibres
attach.
In the first stage of mitosis
the chromosomes copy all
their information and
become shorter and
thicker. They become
known as two chromatids
joined in the centre by a
centromere.
Chromatids get pulled
apart and become
chromosomes.
Chromatids
Centromere
Two new
daughter
cells formed
each
identical to
the parent
cell.
Stages of Mitosis (again we will
learn this properly in Ch10).
Gamete Mother cells
• These are cells found in the testes in men
and ovaries in women that are responsible
for producing gametes (sex cells).
Meiosis (more in Chapter 10)
• This is the process by which a cell called the
gamete mother cell found in the testes in men
and ovaries in women undergo division to form
4 new sex cells ( sperm or egg) each
containing 23 chromosomes.
23
46
92
23
23
23
The gamete mother cell copies
all its information so for a while it
appears to have 92
chromosomes. It undergoes two
meiotic divisions to produce 4
gametes( sex cells) each with 23
chromosomes in them.
Task: Torrance pg 70 Qu’s 1-4
Why do we need DNA
replication?
• When cells divide the daughter cell must
receive an exact copy of the genetic
material from the parent.
• In order for this to happen the DNA must
be replicated or copied.
The importance of DNA
replication
• DNA replication ensures that an exact
copy of the species’ genetic information is
passed from cell to cell during growth and
from generation to generation.
• If DNA failed to replicate itself, the process
of mitosis and meiosis would come to a
halt. DNA replication is therefore essential
for the continuation of life.
DNA UNWINDS AND UNZIPS
DNA helicase unzips the 2 strands
1. Replication starts at a specific sequence on the DNA
molecule. DNA helicase is an enzyme which unwinds
and unzips DNA, breaking the weak hydrogen bonds
that join the base pairs, and forming two separate
strands each with exposed bases.
Free Nucleotides Bind
Complementary base pairing
• DNA helicase completes
the splitting of the strand.
• Meanwhile, free
nucleotides that have been
activated are attracted to
their complementary
bases. Weak hydrogen
bonds are formed between
the bases.
• Each chain acts as a
template.
DNA Polymerase –
Makes the backbone
• Once in place the
activated nucleotides are
joined together by DNA
polymerase.
• DNA polymerase joins
between the sugar of one
nucleotide and the
phosphate of the next by
strong covalent bonds,
forming the phosphatesugar backbone.
Replication Finished
 The result is that there are two DNA molecules, each
with one new synthesised strand of DNA and one
strand from the original.
 The daughter molecules have a base sequence
identical to one another and the original DNA molecule.
 The DNA is then rewound by another enzyme.
Summary: see next diagram!
The 7 Steps of Replication
1.
2.
3.
4.
5.
6.
7.
Original DNA requires replication.
Replication starts at a specific sequence on the DNA molecule.
DNA helicase unwinds and unzips DNA, breaking the hydrogen
bonds that join the base pairs, and forming two separate
strands.
The new DNA is built up from the four nucleotides (A, C, G and
T) that are abundant (free nucleotides) in the nucleoplasm.
These nucleotides attach themselves to the bases on the old
strands by complementary base pairing. Where there is a T
base, only an A nucleotide will bind, and so on.
DNA polymerase joins the new nucleotides to each other by
strong covalent bonds, forming the phosphate-sugar backbone.
A winding enzyme winds the new strands up to form double
helices.
The two new molecules are identical to the old molecule.
DNA Replication
Extension: Not required, FYI:
Semi-conservative Replication
• The model of DNA synthesis (we
have just learned) is also called
Semi-conservative replication.
• Discovered by Meselsohn and
Stahl (1958).
• 2 new strands each containing 1
of the original strands
• Ensures exact copy of genetic
information is passed from cell to
cell and from one generation to
the next enabling continuity of life.
Task: Torrance pg 71 Qu’s 1-4
Essay – 2002
• Give an account of DNA replication under
the following headings:
– DNA structure;
– DNA replication
(7)
(3)
(10)