CHROMOSOMES &

CELL DIVISIONTOPIC 2 - 2015

CHROMOSOMES & CELL DIVISIONThings to cover

ChromosomesKaryotypes

◦ inc. chromosomal disordersCell division:

◦Mitosis◦Meiosis & fertilisation

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

CHROMOSOMES46 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

chromatid

centromere

chromosome

chromosome pair

CHROMOSOMESChromosomes 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)

Sperm…..% are X…..% are Y

X X

XY

Y

XX

XEgg

…..% are X

cell in testis

cell in ovary

1005050

KARYOTYPING

KARYOTYPING

Chromosomes look different from each other:◦eg. shape, size and banding pattern when

stainedThey can be easily sorted out & paired upThis 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.

KARYOTYPINGChromosomal 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)

KARYOTYPINGDown syndrome - Trisomy 21

KARYOTYPINGTurner’s syndrome - Monosomy 23

KARYOTYPINGKlinefelter syndrome – Trisomy 23

KARYOTYPING Cri-du-chat syndrome – Translocation 5

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

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 cellsNew daughter cells are identical to the originalNew cells have the same number of

chromosomes as the parent cell ie. 2 sets of 23 chromosomes = 46 chromosomes

Most body cells replicate this wayRemember – 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 originalNew 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

http://www.youtube.com/watch?v=qCLmR9-YY7o&feature=share&list=EC3EED4C1D684D3ADF