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Reproduction - Edexcel GCSE Biology Revision Notes

Extracts from this document...

Introduction

´╗┐Production of gametes * Meiosis occurs in reproductive systems to produce gametes; the original call contains full number of chromosomes, however the daughter cells are haploid cells so would only have half the original number of chromosomes Without meiosis, the zygote would have 2x the number of chromosome when the gamete fuses during fertilisation. Meiosis produces genetically different gametes. * During meiosis the DNA replicates so there?re two identical copies of each chromosome, called chromatids. Then they condense to form double stranded Chromosomes, again consisting of 2 sister chromatids connected at the centromere. Chromosomes arrange themselves into homologous pairs ? pairs of matching chromosomes (i.e. both are chromosome no. 1) each of the chromosome in a pair is form each parent, paternal and maternal. During 1st division, homologous pairs are separated, halving the number of chromosomes and in the 2nd division, the pair of sister chromatids are separated at the centromere (by spindle fibres) ? this halves the no. of chromosomes again. * Genetic variation refers to the differences that exists between individuals? genetic materials (different combination of alleles) and meiosis creates this variation: CROSSING OVER OF CHROMATIDS * Before the 1st division in meiosis, homologous pairs (maternal and paternal version of the same chromosome) ...read more.

Middle

and into the embryo sac within the ovule. * In the embryo sac, the tube nucleus disintegrates and the tip of the pollen tube bursts, releasing the two male nuclei into the sac. * One male nucleus fuses with the egg nucleus to make a zygote. The zygote divides by mitosis to form the embryo of the seed. * Whereas the second male nucleus fuses with two other nuclei (called the polar nuclei) at the centre of the embryo sac - producing a cell with a large nucleus, which divides to become a food store (endosperm - 3n) for the mature seed. * A double fertilization has taken place and this only happens in flowering plants. Production of gametes * Meiosis occurs in reproductive systems to produce gametes; the original call contains full number of chromosomes, however the daughter cells are haploid cells so would only have half the original number of chromosomes Without meiosis, the zygote would have 2x the number of chromosome when the gamete fuses during fertilisation. Meiosis produces genetically different gametes. * During meiosis the DNA replicates so there?re two identical copies of each chromosome, called chromatids. Then they condense to form double stranded Chromosomes, again consisting of 2 sister chromatids connected at the centromere. Chromosomes arrange themselves into homologous pairs ? pairs of matching chromosomes (i.e. ...read more.

Conclusion

the 2 haploid nuclei fuses (this is fertilisation) and a zygote is formed - mitosis immediately begins. Plant Fertilisation * A pollen grain lands on the stigma of a flower, the grain absorbs water and splits open. A pollen tube grows out of the grain down the style. - There are 3 nuclei in the pollen tube, 2 male gamete nuclei behind 1 tube nucleus at the tip of the tube which makes enzyme that digests surrounding cells, making a way through for the pollen tube. * When the tube reaches the ovary, it grows through the micropyle (which is a tiny hole in the ovule wall) and into the embryo sac within the ovule. * In the embryo sac, the tube nucleus disintegrates and the tip of the pollen tube bursts, releasing the two male nuclei into the sac. * One male nucleus fuses with the egg nucleus to make a zygote. The zygote divides by mitosis to form the embryo of the seed. * Whereas the second male nucleus fuses with two other nuclei (called the polar nuclei) at the centre of the embryo sac - producing a cell with a large nucleus, which divides to become a food store (endosperm - 3n) for the mature seed. * A double fertilization has taken place and this only happens in flowering plants. ...read more.

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