Topic 10 - Further genetics Flashcards
What happens in meiotic prophase I?
- Chromosomes become more visible because they become shorter and thicker
- Homologous chromosomes pair up, maternal and paternal chromosomes are side by side. Each pair is called a bivalent
- Crossing over occurs
- Centrioles move to the poles (in animal cells)
- Nucleoli break down
- At the end the nuclear membrane breaks down too
What happens in meiotic metaphase I?
- Chromosomes continue to shorten and thicken
- Spindle microtubules attach to the centromeres
- Bivalents line up randomly on the equator → random orientation
- Crossing over is terminated
- Chiasmata slide towards the ends of the chromosomes, causing the shapes of the bivalents to change
- At the end the chromosomes start to move
What happens in meiotic anaphase I?
- Homologous chromosomes are pulled to opposite poles. This halves the chromosome number
- At the end of the chromosomes reach the poles
What happens in meiotic telophase I?
- Nuclear membranes form around the chromosomes at each pole
- The chromosomes uncoil partially
- Cytokinesis occurs to form two haploid cells (chromosomes remain undivided)
- At the end the cells enter a brief period of interphase or immeadiately proceed to phase 2
What happens in meiotic prophase II?
- Chromosomes become shorter and thicker again by coiling
- Chromosomes are not in bivalents
- Centrioles move to the poles
- At the end the nuclear membranes break down
What happens in meiotic metaphase II?
- Spindle microtubules attach to the centromeres of sister chromatids
- Chromosomes line up on the equator randomly
- At the end the centromeres divide
What happens in meiotic anaphase II?
- Each chromatid becomes a chromosome
- The two chromosomes are pulled to opposite sides by microtubules
- Due to random orientation, the chromosomes can be pulled to either pole
- At the end the chromosomes reach the poles
What happens in meiotic telophase II?
- Nuclear membranes form around the chromosomes at each pole
- Cytokinesis occurs to form four cells in total
- The chromosomes uncoil and become invisible
- Nucleoli appear
- Cells formed at the end of telophase II develop into gametes
How are chiasmata formed in the process of crossing over?
- The bivalents line up. Bivalents are the same length, have their centromeres in the same position, and contain the same genes stored at the same loci.
- The chromatids intertwine and break at exactly the same position in adjacent non-sister chromatids
- The separated tips connect to the corresponding position on the other chromatid. Each chromatid has a segment of the other’s genetic material
- Many chiasmata are formed during crossing over at the same time
How does meiosis result in an infinite genetic variety in gametes?
Stages responsible for creating genetic variety are crossing over in prophase I and random orientation in metaphase I
- Due to crossing over the two sister chromatids are no longer identical to each other, each one has different combinations of maternal and paternal alleles
- Also random orientation
What is 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
What is the relaitonship between Mendel’s law of independent assortment and meiosis?
- If pairs of genes are located on different chromosome
- Random orientation allows combinations of allels to be broken up, so that new combinations can be formed when gametes fuse during fertilisation
Distinguish between autosomes and sex chromosomes
Any chromosome which is not a sex chromosome is called an autosome
How can crossing over result in an exchange of alleles?
If an allele has its locus on some point of a chromosome and crossing over occurs above this point, the allele switches chromosomes. A single bivalent can have several chiasmata producing crossing over in more than one chromatid.
What is a linkage group
Any two genes which are found on the same chromosome
Identify which of the offspring are recombinants in a dihybrid cross involving linked genes
