Meiosis- Biology

• Meiosis produces four daughter nuclei each with half the number of chromosomes of the parent.
• The daughter cells are genetically different.
• Meiosis contributes to heritable variation by allowing sexually produced organisms to vary in characteristics.
• Each of the cells of an adult of a species has a fixed number of chromosomes, usually made up of pairs that control the same characteristics.
• One of each pair is derived from the chromosomes provided in the egg by the mother (maternal chromosomes) and the other of each pair is derived from the chromosomes provided in the sperm by the father (paternal chromosomes).
• These are known as homologous pairs and the total number is referred to as the diploid number (2n); in humans this is 46.
• During meiosis each of the pairs are separated, so that only one enters each sex cell or gamete.
• The separation of homologous chromosomes during meiosis is known as the haploid number (n) and in humans is 23.
• When two haploid gametes fuse, the diploid number of chromosomes is restored.
• The new cell formed by the fusion of two haploid gametes is called a zygote and this then divides by mitosis to maintain the diploid number in all further cells.
• A karyotype is the number and appearance of chromosomes in the nucleus of a eukaryotic cell.
• During prophase of mitosis each chromosome is made up of two identical structures called chromatids.
• Homologous chromosomes move to the same pole during nuclear division.
• Homologous chromosomes have the same sequence of base pairing in their DNA structure.
• Only diploid cells contain homologous chromosomes.
• Independent assortment during meiosis means that all of the maternal or paternal chromosomes will not be separated into one cell.
• Crossing over is the exchange of alleles between chromatids of homologous chromosomes as a result of chiasma formation during prophase of meiosis I.
• Random fertilisation means that any male gamete could fuse with any female gamete.
• Structural changes on chromosomes as a result of mutations, the pairing of Y and X chromosomes, and chiasmata formation in prophase are not associated with metaphase 1 of meiosis.
• The bivalents move to the equatorial plate across the centre of the cell during metaphase 1 of meiosis.
• The paternal and maternal chromosomes in each bivalent position themselves independently of the others during metaphase 1 of meiosis.
• The chromosomes are pulled towards the poles by the shortening of the microtubules during metaphase 1 of meiosis.
• The microtubules attach to the centromere of each chromosome during metaphase 1 of meiosis.
• Meiosis contributes to the heritable variation by allowing sexually produced organisms to vary in characteristics.