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
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 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 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 X Egg …..% are X cell in testis cell in ovary 100 50
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)
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: ◦ G rowth ◦ A sexual reproduction ◦ T issue repair and ◦ E mbryonic 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!)
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