CHROMOSOMES CELL DIVISION TOPIC 2 2015 CHROMOSOMES CELL

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. 8 m long and is squeezed into the nucleus which is only 0. 00001 m 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) cell in testis XX XY 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 2 Xs, 1 Y (47, XXY) ◦ Cri-du-chat syndrome = TRANSLOCATION on Ch 5 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 – Mi. TOsis 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.

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