meiosis 6.1.2

• a chromosome is a thread like structure made of dna and found in the nucleus
• mot human cells contain 46 chromosomes (23 pairs), half from the mother and half from the father
• exceptions include red blood cells which have 0 chromosomes
• 23 chromosomes come from the mothers egg and 23 come from the fathers sperm
• meiosis occurs in the ovaries in women
• meiosis occurs in testes in men
• gonads are reproductive organs
• cells which have 46 chromosomes are called diploid (most body cells)
• cells which have 23 chromosomes are haploid (egg and sperm cells)
• meiosis

  1. a cell in the reproductive organs looks like a normal body cell before it starts to divide and form gametes
• 2) as in normal cell division, the first step is that the chromosomes are copied
• 3) the cell divides in 2 and these new cells immediately divide again
• 4) this ends in four sex cells (gametes), each with a single set of chromosomes in it. each of the gametes is genetically different from the others because the chromosome are all shuffled up and given at random
• during fertilisation the sperm and the egg cells fuse to form a zygote
• it is essential that gametes are haploid because because the 23 chromosomes from each combine to give 46 in the new cell, if the gametes werent haploid then the number of chromosomes of the zygote would be double that of the parents
• after fertilisation the zygote can then divide further by mitosis to form an embryo
• genetic variation occurs in gametes because each gamete that is produced is genetically different from all the others. gametes contain random mixtures of the original chromosomes, this introduces variation
• every fertilised ovum is genetically unique because every ovum is made from 2 gametes which are all genetically different through the process of meiosis which makes them unique
• meiosis produces 4 daughter cells but mitosis produces 2
• meiosis daughter cells have 23 chromosomes, but mitosis daughter cells have 46 chromosomes
• meiosis daughter cells are haploid (1 copy of each chromosome) but mitosis daughter cells are diploid (2 copies of each chromosome)
• meiosis produces daughter cells that are genetically different to the parent cells but mitosis produces daughter cells that are genetically identical to each other and the parent cells
• meiosis happens in reproductive organs whereas mitosis happens in normal body cells
• meiosis is for creating a child but mitosis is for growth of new cells, repair and replacement of worn out cells and asexual reproduction in plants and yeast