Post on 24-Dec-2015
transcript
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.
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