Meiosis The formation of gametes Meiosis Most organisms

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Meiosis The formation of gametes.

Meiosis The formation of gametes.

Meiosis • Most organisms use sexual reproduction in at least part of their life

Meiosis • Most organisms use sexual reproduction in at least part of their life cycle. • Sexual reproduction involves the fusion of nuclei from 2 different parents. • The offspring end up with a unique combination of genes. • For this to work there needs to be a way of reducing the number of chromosomes in sex cells before reproduction.

Meiosis • If this did not happen, the numbers of chromosomes would double every

Meiosis • If this did not happen, the numbers of chromosomes would double every generation. • Meiosis is the cell division process that ensures the halving of chromosome numbers.

Meiosis Adult ♀ Fertilisation Mitosis Egg Zygote → embryo→ Foetus→ Baby→ Adult ♂ Sperm

Meiosis Adult ♀ Fertilisation Mitosis Egg Zygote → embryo→ Foetus→ Baby→ Adult ♂ Sperm

Meiosis • • Cell division which has 2 stages Halves the chromosome number Used

Meiosis • • Cell division which has 2 stages Halves the chromosome number Used to form gametes/ sex cells Produces 4 gametes, each with a haploid number of chromosomes.

Meiosis I • Interphase I – Same as in Mitosis. – DNA replicates, giving

Meiosis I • Interphase I – Same as in Mitosis. – DNA replicates, giving 2 identical chromatids joined at a centromere. – The centrioles replicate and move to each end of the nucleus. – The nucleolus disappears, as does the nuclear membrane.

Meiosis I • Prophase I – Homologous sets of sister chromatids undergo synapsis. –

Meiosis I • Prophase I – Homologous sets of sister chromatids undergo synapsis. – They line up side by side, attached by their centromeres, to form a tetrad (4 chromatids). – During synapsis the chromatids may exchange pieces of DNA – a process called Crossing Over. – The points on the chromatids where they break and rejoin are called Chiasmata. – The centrioles form a spindle across the nuclear material.

Crossing Over £This is where a piece of one chromosome is exchanged for a

Crossing Over £This is where a piece of one chromosome is exchanged for a piece of its homologous pair. £This gives rise to new combinations of genes. £The points on the chromatids where they break and rejoin are called Chiasmata. (singular – chiasma)

Meiosis I • Metaphase I – The tetrads line up across the equator of

Meiosis I • Metaphase I – The tetrads line up across the equator of the spindle. – Spindle fibres from one of the poles attach to one chromosome of each pair. – Spindle fibres from the opposite pole attach to the homologous chromosome.

Meiosis I • Anaphase I – The centromeres linking the tetrads separate. – The

Meiosis I • Anaphase I – The centromeres linking the tetrads separate. – The pairs of chromatids move as single units towards the poles.

Meiosis I • Telophase I – The two daughter cells form and the nuclear

Meiosis I • Telophase I – The two daughter cells form and the nuclear membrane re-forms. – Each cell now has a haploid set of chromosomes. – BUT each chromosome still has 2 chromatids. – Often the cytoplasm also divides by cytokinesis before the cell goes into the second division phase, called meiosis II

Meiosis II • This cell division proceeds in a similar manner to mitosis, except

Meiosis II • This cell division proceeds in a similar manner to mitosis, except there is no DNA replication beforehand. • Prophase II – A spindle forms at right angles to the previous spindle. – The chromosomes move to the equator of each cell.

Meiosis II • Metaphase II – The chromosomes line up at the equator as

Meiosis II • Metaphase II – The chromosomes line up at the equator as in mitosis.

Meiosis II • Anaphase II – The centromeres of sister chromatids separate. – The

Meiosis II • Anaphase II – The centromeres of sister chromatids separate. – The sister chromatids of each pair are now individual chromosomes. – These move towards opposite poles of the cell.

Meiosis II • Telophase II – Nuclei begin to form at opposite poles, and

Meiosis II • Telophase II – Nuclei begin to form at opposite poles, and cytokinesis occurs.

Meiosis II • At the end of meiosis, cells have half the chromosome number.

Meiosis II • At the end of meiosis, cells have half the chromosome number. • There are four cells, which are not all genetically identical, especially if there has been crossing over.

Gametogenesis • Meiosis is different in males and females. • In males its called

Gametogenesis • Meiosis is different in males and females. • In males its called Spermatogenesis. • In Females its called Oogenesis.

Spermatogenesis • In males, each cell formed becomes a sperm cell. • The sequence

Spermatogenesis • In males, each cell formed becomes a sperm cell. • The sequence is uninterrupted. • Adult males produce as many as 250 – 400 million sperm cells in each ejaculation.

Oogenesis • The formation of eggs differs in 3 ways: – During the first

Oogenesis • The formation of eggs differs in 3 ways: – During the first meiotic division cytokinesis is uneven – one cell takes all the cytoplasm going on to form the egg. The smaller cell forms polar bodies which disintegrate. – At birth the ovary has all the cells it will ever have to develop into eggs.

Oogenesis – Formation of the eggs includes long “resting” periods. Between birth and puberty,

Oogenesis – Formation of the eggs includes long “resting” periods. Between birth and puberty, the egg cells enlarge and the follicles around them grow. The primary egg cells replicate their DNA. – They enter prophase I but do not change again until activated by hormones at puberty. – An egg cell activated periodically by hormones completes meiosis I, then stops. – Penetration of the sperm triggers meiosis II.