Topic 10.1 Meiosis Flashcards
10.1.1 Describe the behaviour of the chromosomes in the phases of meiosis.
Prophase I
- Chromosomes start to coil up and so become shorter and thicker
- Homologous chromosomes pair up
- Crossing over occurs
- Centrioles move to poles in animal cells
- Nucleoli break down
- Nuclear membrane breaks down at end of prophase I
Metaphase I
- Chromosomes continue to shorten and thicken
- Spindle microtubules attach to centromeres
- Bivalents (pairs of homologous chromosomes) line up on the equator
- Chiasmata slide towards ends of chromosomes, causing shapes of bivalents to change
- Chromosomes start to move at end of metaphase I
Anaphase I
- 2 chromosomes of each bivalent move to opposite poles (this halves chromosome #)
- Each chromosome = 2 chromatids; because of crossing over, they aren’t identical
Telophase I
- Nuclear membranes form around groups of chromosomes at each pole
- Cell divides to form 2 haploid cells
- Chromosomes uncoil partially
- At end of telophase I, 2 cells either enter brief period of interphase or immediately proceed to meiosis II; DNA isn’t replicated
Prophase II
- Chromosomes become shorter & thicken again by coiling
- Centrioles move to poles in animal cells
- Nuclear membranes break down at end of prophase II
Metaphase II
- Spindle microtubules attach to centromeres
- Chromosomes line up on the equator
- Centromeres divide at end of metaphase II
Anaphase II
- 2 chromatid of each chromosome move to opposite poles
- Chromatids reach poles at end of anaphase II
Telophase II
- Nuclear membranes form around groups of chromatids (now considered chromosomes) at each pole
- 2 cells each divide to form 4 cells in total
- Chromosomes uncoil
- Nucleoli appear
- In most organisms, cells former at end of telophase II develop into gametes
10.1.2 Outline the formation of chiasmata in the process of crossing over.
- At one stage in prophase I all of the chromatids of two homologous chromosomes become tightly paired up together. This is called synapsis.
- DNA molecule of one of the chromatids is cut.
- Second cut is made at exactly the same point in the DNA of a non-sister chromatid.
- The DNA of each chromatid is joined up to the DNA of the non-sister chromatid
- This has the effect of swapping sections of DNA between the chromatids.
- In the later stages of prophase I the tight pairing of the homologous chromosomes ends, but the sister chromatids remain tightly connected. When each cross-over has occurred there is an X- shaped structure called a chiasma (point where cross over occurs, holds homologous chromosomes together in bivalents)
10.1.3 Explain how meiosis results in an effectively infinite genetic variety in gametes through crossing over in prophase I and random orientation in metaphase I.
An almost infinite amount of genetic variety is produced, as a result of crossing-over in prophase I and the random orientation of bivalents in metaphase I.
10.1.4 State Mendel’s law of independent assortment.
When gametes are formed, the separation of one pair of alleles between the daughter cells is independent of the separation of another pair of alleles. e.g. eye colour separate from hair colour
10.1.5 Explain the relationship between Mendel’s law of independent assortment and meiosis.
- Similar to how the separation of one pair of alleles is independent of the separation of another pair (Mendal’s Law of Independent Assortment):
- In meiosis, when homologous chromosomes pair up (bivalents), the genes are on different pairs
- The bivalents are oriented randomly on the equator, so the pole to which alleles of the bivalent move aren’t affected by the poles which other bivalents move towards
