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level: 9.2 Meiosis and genetic variation

Questions and Answers List

level questions: 9.2 Meiosis and genetic variation

QuestionAnswer
Gametes (zi-goat)- Sperm cells in men, egg cells in women - Join together at fertilisation to form zygote, which divides into a new organism
Diploid number- Normal body cells have a diploid no. (2n) of chromosomes - Meaning each cell has 2 of each chromosome, 1 from mum 1 from dad
Haploid- Gametes have haploid (n) no. of chromosomes - There's only 1 copy of each chromosome
Fertilisation- Any sperm can fertilise any egg, random - Produces zygotes with different combinations of chromosome to both parents - Mixing of genetic material in sexual reproduction increases genetic diversity within species
During fertilisation- Haploid sperm fuses with haploid egg - Making cell with normal diploid no. of chromosomes - 1/2 these chromosomes from sperm, 1/2 from egg
Meiosis- Type of cell division - Takes place in reproductive organs - Cells divide by meiosis are diploid at start - Haploid at end, chromosome no. halves - Without it, you'd get double no. of chromosomes, not good
Meiosis process (1)- Before meiosis starts, DNA unravels + replicates - So there's 2 copies of each chromosomes, chromatids
Meiosis process (2)- DNA condenses, forming double-armed chromosomes - Each made from 2 sister chromatids - Sister chromatids joined in middle by centromere
Meiosis process (3)- Meiosis I (first division) - Chromosomes arrange themselves into homologous pairs
Meiosis process (4)- These homologous pairs are then separated, halving the chromosome no.
Meiosis process (5)- Meiosis II (second division) - Pairs of sister chromatids that make up each chromosome are separated (the centromere is divided)
Meiosis process (6)- Produces 4 haploid cells (gametes) that are genetically different
+ Meiosis process (1)- Interphase, chromosomes replicate, but not yet visible - Prophase I, chromosomes condense become visible - Late prophase I, HC arrange themselves in pairs
+ Meiosis process (2)- Metaphase I, HP line up on the equator of spindle - Anaphase I, HC separate - Telophase I, two daughter nuclei formed - Cytokinesis I, cytoplasm divides
+ Meiosis process (3)- Prophase II, chromosomes become visible again - Metaphase II, individual chromosomes line up on new spindle
+ Meiosis process (4)- Anaphase II, chromosomes separating - Telophase II, 4 haploid nuclei formed - Cytokinesis II, cytoplasm divides to form 4 haploid (gamete) cells
Main events during meiosis that lead to genetic variation- Crossing over chromatids - Independent segregation of chromosomes
Crossing over chromatids- Crossing over of chromatids in meiosis I means that the 4 daughter cells have chromatids with different alleles - Increasing genetic variation
Process of crossing over (1)- The chromatids of each pair become twisted around 1 another
Process of crossing over (2)- During this twisting process tensions are created - Proportions of the chromatids break off
Process of crossing over (3)- These broken portions might then re-join with the chromatids of its homologous pair, recombination
Process of crossing over (4)- Usually it is same portions of HCs that are exchanged
Process of crossing over (5)- Produces new genetic combinations of maternal + paternal alleles
Independent segregation of chromosomes- Each pair of homologous chromosomes in cells made up of 1 chromosome from mum + 1 from dad - When homologous pairs are separated in meiosis I, which chromosome ends up in which daughter cells is random - So 4 daughter cells have completely different combinations of those maternal + paternal chromosomes - Leads to genetic variation
Chromosome combinations in meiosis calculations2^n where n = no. of pairs of homologous chromosomes (2^n)² where n = no. of pairs of homologous chromosomes
Differences between mitosis + meiosis (MITOSIS)- Produces cells with same no. of chromosomes as parent cell - Daughter cells are genetically identical to each other + parent cell - Produces 2 daughter cells
Differences between mitosis + meiosis (MEIOSIS)- Produces cells with 1/2 the no. of chromosomes as parent cell - Haploid cells are genetically different from each other + parent cell - Produces 4 haploid cells
+ Independent assortment of homologous chromosomes- HCs are randomly distributed to daughter cells, and different chromosomes segregate independently of each other - So gametes have unique combinations of chromosomes - Met 1
+ Crossing over of chromatids- Pro 1 - Starts before the chromosomes condense - HCs held together to form a bivalent - Crossing point of sister chromatids is called a chiasma - Leads to exchange of alleles
+ Explain how meiosis leads to genetic variation of offspring?- Independent assortment of HC - Met 1 - Independent assortment of sister chromatids - Met2 - Crossing over of HC (Pro 1) - Forming recombinants - By exchanging alleles