CHROMOSOMES
&
CELL DIVISION
TOPIC 2 - 2015
CHROMOSOMES
& CELL DIVISION
Things to cover
Chromosomes
 Karyotypes

◦ inc. chromosomal disorders

Cell division:
◦ Mitosis
◦ Meiosis & fertilisation
CHROMOSOMES
CHROMOSOMES
When the cell is preparing to divide, its DNA becomes
tightly coiled into chromosomes.
 It needs to coil so that it fits into the nucleus!
 Human DNA is ~1.8m long and is squeezed into the
nucleus which is only 0.00001m wide!
 Multiply that by the 50-75 trillion cells in the body!!!

CHROMOSOMES

46 chromosomes can be found within the
nucleus of every cell.

The chromosomes
are in 23 pairs.
These pairs are
called homologous
chromosomes.
CHROMOSOMES

Each chromosome is made of two strands
called chromatids.

The two chromatids are held together at the
centromere.

These chromosomes are only visible when a
cell is about to divide or in the process of
dividing.
CHROMOSOMES
chromosome
chromatid
centromere
chromosome pair
CHROMOSOMES Chromosomes on
the move during
cell division
CHROMOSOMES

Of the 23 pairs:
◦ 23 chromosomes (1 set) come from dad’s
sperm cell
◦ 23 chromosomes (1 set) come from mum’s
ovum / egg cell
CHROMOSOMES

Of the 23 pairs:
◦ 22 pairs are autosomes:
 code for non-sexual characteristics
◦ 1 pair are sex chromosomes (allosomes):
 code for both sexual and non-sexual
characteristics
 can be either a longer X chromosome or a
shorter Y chromosome
CHROMOSOMES
CHROMOSOMES

The sex chromosomes are the only pair
that may not match
◦ Females have two X chromosomes (XX)
◦ Males have an X and a Y chromosome (XY)
XX
XY
cell in
testis
X
Sperm
…..% are X
50
50
…..% are Y
Y
X
cell in
ovary
X
Egg
100
…..% are X
KARYOTYPING
KARYOTYPING

Chromosomes look different from each other:
◦ eg. shape, size and banding pattern when
stained

They can be easily sorted out & paired up

This process is called karyotyping.

Dividing cells are treated, stained, put on slides
and photographed, cut up and arranged into
pairs.

It is used by geneticists to investigate
chromosomal disorders.
KARYOTYPING
Chromosomal disorders

Disorders arise when there are extra
chromosomes or fewer chromosomes.

For example:
◦ Down syndrome = TRISOMY 21
3 copies of chrom.21 (47,XX or 47, XY)
◦ Turner’s syndrome = MONOSOMY 23
only one X (45,X)
◦ Klinefelter syndrome = TRISOMY 23
2Xs, 1Y (47,XXY)
◦ Cri-du-chat syndrome = TRANSLOCATION on Ch5
A deletion of an arm from chrom.5 (46, XX or 46, XY)
KARYOTYPING
Down syndrome - Trisomy 21
KARYOTYPING
Turner’s syndrome - Monosomy 23
KARYOTYPING
Klinefelter syndrome – Trisomy 23
KARYOTYPING
Cri-du-chat syndrome – Translocation 5
CELL DIVISION
CELL DIVISION

The body’s cells need to divide in order
to make the organism grow.

There are 2 types of cell division:
1. Mitosis
2.Meiosis
CELL DIVISION
Mitosis

Mitosis is used for:
◦ Growth
◦ Asexual reproduction
◦ Tissue repair and
◦ Embryonic development
Remember: GATE
CELL DIVISION
Mitosis

Forms 2 new cells

Starts with a body cell and produces body cells

New daughter cells are identical to the original

New cells have the same number of
chromosomes as the parent cell
ie. 2 sets of 23 chromosomes = 46 chromosomes

Most body cells replicate this way

Remember – MiTOsis for TOES!
CELL DIVISION
Meiosis

Forms 4 new cells

Starts with a body cell and produces sex cells
(gametes)

Daughter cells are not identical to the original

New cells have half the number of chromosomes
as the parent cell (ie. 23 chromosomes)

Meiosis prevents doubling of
the chromosomes
at fertilisation
CELL DIVISION
Meiosis

Human gametes only have 23 chromosomes.

Gametes are said to be haploid cells – containing
half the chromosome number of the normal
diploid body cells.

This means that they have only one chromosome
from each homologous pair in the parent cell.
CELL DIVISION
Meiosis

The gametes produced are unique.
Why is it important to produce genetically unique
gametes?

It ensures natural variation within a species.

It means that siblings may
share some characteristics
but will not be identical
(unless they are twins!)
CELL DIVISION
FERTILISATION
FERTILISATION




Fertilisation is the fusion of gametes.
When an egg is fertilised by a sperm, it becomes a
zygote.
This zygote divides by mitosis many times and
becomes an embryo.
The embryo grows and develops into a foetus.
Please note:
This type of
slide is a
hyperlinked
video