6.3- Meiosis Flashcards
What is meant by the terms diploid, gametes, zygote?
Diploid - the normal chromosome number: Normal cells have 2 chromosomes of each type. One from each parent.
In sexual reproduction, two sex cells (gametes) fuse to produce a fertilised egg (zygote).
Gametes must contain half the standard (diploid) number of chromosomes.
Explain through which process gametes are formed.
Gametes are formed by another form of cell division called meiosis.
- the nucleus divides twice to form 4 daughter cells= the gametes.
- each gamete contains half of the chromosome number of the parent cell. It is haploid.
- meiosis is therefore known as reduction division.
What are homologous chromosomes?
Each nucleus contains matching sets of chromosomes, called homologous chromosomes, and is termed a diploid.
- each chromosome in a homologous pair has the same genes at the same loci.
- as homologous chromosomes have the same genes in the same positions, they will be the same size and length once visible in prophase. Centromeres will also be in the same positions.
What are alleles?
Different versions of the same gene.
The different alleles of a gene will have the same locus (position on particular chromosome)
What are the stages of meiosis?
involves 2 divisions:
1. Meiosis I - the reduction division when the pairs of homologous chromosomes are separated into 2 cells. The cells are haploid as each new cell will only contain one full set of genes instead of two.
2. Meiosis II - the pairs of chromatids present in the daughter cells are separated to form 2 more cells.
4 haploid daughter cells produced in total.
Explain prophase 1
- chromosomes condense.
- nuclear envelope disintegrates.
- nucleolus disappears.
- spindle formation begins.
- homologous chromosomes pair up, forming bivalents.
- crossing over occurs as chromosomes are large and chromatids entangle whilst moving through the liquid cytoplasm.
Explain metaphase 1.
- homologous pairs of chromosomes assemble along the metaphase plate.
- orientation of each homologous pair on the metaphase plate is random and independent.
- independent assortment = maternal/paternal chromosomes end up facing either pole resulting in different combinations of alleles. This results in genetic variation.
Explain anaphase 1.
- homologous chromosomes are pulled to opposite poles and chromatids stay joined together.
- sections of dna on ‘sister’ chromatids which entangled during cross over, break off and rejoin. Sometimes dna is exchanged as a result.
Points at which the chromatids break and rejoin = chiasmata. - the exchange forms recombinant chromatids. Combination of alleles on them will be different from combination on original chromatids.
- genetic variation rises from new combination. Sister chromatids = no longer identical.
Explain telophase 1.
- chromosomes assemble at each pole.
- nuclear membrane reforms.
- chromosomes uncoil.
- cell undergoes cytokinesis and divides into 2 cells.
- reduction of chromosome number from diploid to haploid is complete.
Explain prophase 2.
- chromosomes(still consisting of 2 chromatids) condense and become visible again.
- nuclear envelope breaks down.
- spindle formation begins.
Metaphase 2.
- individual chromosomes assemble on the metaphase plate.
- due to crossing over, the chromatids are no longer identical = independent assortment again = more genetic variation.
Anaphase 2.
- chromatids of individual chromosomes are pulled to opposite poles after division of the centromeres.
same as mitosis anaphase.
Telophase 2.
- chromatids assemble at the poles.
- chromosomes uncoil and form chromatin again.
- nuclear envelope reforms.
- nucleolus becomes visible.
- cytokinesis results in division forming 4 daughter cells.
Cells haploid due to reduction division.
Cells are genetically different to eachother + parent cell due to crossing over/independent assortment.
