What is the link between
these images?
What is the
common link?
Down’s Syndrome can be caused
where sufferers have three copies of
chromosome 21.
Trisomy 13, also called Patau
syndrome, is a disorder in
which an individual has three
copies of genetic material from
chromosome 13, rather than
two.
Jacobsen syndrome results
from a loss of genetic material
from the end of the long arm of
chromosome 11.
Sufferers of Klinefelter’s
syndrome are born with
an extra X
chromosome, making
them XXY.
What is the
common link?
CHROMOSOMES
DNA and chromosomes
Aims:
•How does DNA in
prokaryotic organisms differ
from the DNA in eukaryotic
organisms?
•What is a chromosome?
•How are genes arranged on
a DNA molecule?
A eukaryote is an organism whose cells
contain a nucleus and other structures
(organelles) enclosed within membranes
The prokaryotes are a group of
organisms whose cells lack a
membrane-bound nucleus
Eukaryote
DNA larger
Linear
Associated with
proteins
Form
Chromosomes
Prokaryote
DNA smaller
Circular
Not associated
with proteins
Plasmids
https://www.youtube.com/watch?v=q
oERVSWKmGk




Only visible during cell division.
Each chromosome is one
molecule of DNA
Each species has a specific
number of chromosome eg
Humans :46 or 23 pairs
Daughter cells contain an
exact copy of the instructions
in the parent cell –they are
genetically identical
Humans have 23 pairs of chromosome




When visible, the DNA has
replicated itself and
chromosomes appear as two
threads, joined at a single
point called the centromere.
Each replicated thread is
called a chromatid.
The DNA replicas are called
sister chromatids
Sister chromatids end up in
different daughter cells


The DNA in chromosomes is held in place by
proteins.
The molecules of DNA are wrapped around
proteins called histones. (over 50%)

DNA + histone are called chromatin fibres

DNA carries the instructions
A nucleosome
DNA Helix
Histone
protein
Core of 8 histone proteins
Coiled
Chromatin fibres
Chromosome




The chromatin is coiled up to form
supercoiled chromosomes
The supercoiled chromosomes can be
moved around the cell during mitosis
Supercoiled chromosomes can be
stained and seen under the light
microscope – 500nm thick
As supercoiled chromosomes they
cannot code mRNA so they remain as
such for as little time as possible
http://www.youtube.com/watch?v=9kQpYdCnU1
4
Understanding checkpoint
Turn to your partner.
Number 1’s:
Explain in as much detail as possible:
What is a chromosome?
Number 2’s:
Define and explain the following key
words:
1.Histone
2.Coiling
3.Chromosome
4.Chromatid




Chromosomes occur in pairs called
homologous chromosomes
A homologous pair is always two
chromosomes that determine the same
genetic characteristics
One of the pair is derived from the mother in
the egg (maternal chromosome)
The other is provided by the father in the
sperm (paternal chromosome)


The genes that code for a particular
polypeptide are usually found at the same
position in the DNA molecule of their
chromosome in every cell in the body This
position is called the locus (plural loci).
In some cases the two genes that code for a
particular polypeptide have exactly the same
code. In others they are different although
they still occur at the same position on the
chromosome. These different forms of the
same gene are called alleles
8.3
DNA and chromosomes
Plenary
1: What is a gene?
2: What are homologous
chromosomes?
3:What is a polypeptide?
4: What is a histone?
5: What is chromatid?
6: Name 2 things you have
learned today.