Meiosis

Question 0

What happens in prophase 1 in meiosis?

Answer 0

• chromatin condenses and undergoes supercoiling
• chromosomes come together in their homologous pairs to form a bivalent
• non sister chromatids wrap around each other at chiasmata
• they may undergo crossing over
• nucleolus disappears and nuclear envelope disintegrates
• a spindle forms
• can last days months or years depending on the species

Question 1

What is meiosis?

Answer 1

It is a reduction division. The resulting daughter cells have half the original numbers of chromosomes and they are haploid

Question 2

What happens during metaphase 1 of meiosis?

Answer 2

• bivalent a line up across the equator of the spindle, attached to spindle fibres at the centromeres
• the bivalents are arranged randomly with each member of a homologous pair facing opposite poles
• this allows the chromosomes to independently segregate when they are pulled apart in anaphase 1.

Question 3

What happens in anaphase 1 in meiosis?

Answer 3

• the homologous chromosomes in each bivalent are pulled by the spindle fibres to opposite poles
• the centromeres do not divide
• the chiasmata separate and lengths of chromatid that have been crossed over remain with the chromatid to which they have become newly attached.

Question 4

What happens in telophase 1 of meiosis?

Answer 4

• in most animal cells 2 new nuclear envelopes form, 1 around each set of chromosomes at each pole, and the cell divides by cytokinesis. There is a brief interphase and the chromosomes uncoil
• in most plant cells the cell goes strait from anaphase 1 into meiosis 2

Question 5

How does meiosis 2 occur in relation to meiosis 2?

Answer 5

At right angles to it

Question 6

What occurs in prophase 2 of meiosis?

Answer 6

• if a nuclear envelope has reformed, it breaks down again

* the nucleolus disappears, chromosomes condense and spindles form

Question 7

What happens in metaphase 2 of meiosis?

Answer 7

• the chromosomes arrange themselves on the equator of the spindle. They are attached to the spindle fibres at the centromeres
• the chromatids of each chromosome are randomly assorted

Question 8

What occurs in anaphase 2 of meiosis?

Answer 8

•the centromeres divide and the chromatids are pulled to the opposite poles by the spindle fibres. The chromatids randomly segregate

Question 9

What happens in telophase 2 in meiosis?

Answer 9

• nuclear envelopes reform around the haploid daughter nuclei
• in animals, the 2 cells now divide to give 4 hasp loud cells
• in plants, a tetras of 4 haploid cells is formed.

Question 10

Define maternal chromosomes

Answer 10

The set of chromosomes in an individual’s cells that were contributed by the egg

Question 11

Define paternal chromosomes

Answer 11

The set of chromosomes in an individual’s cells that were contributed by the sperm

Question 12

How does meiosis and fertilisation lead to genetic variation?

Answer 12

• crossing over shuffles alleles
• genetic reassortment due to random distribution and segregation of maternal and paternal chromosomes
• genetic reassortment due to random distribution and segregation of sister chromatids
• random mutation

Question 13

Describe the process of crossing over

Answer 13

• Non-sister chromatids wrap around each other and attach at chiasma
• at these points the chromatids may break and rejoin in the same bivalent, this leads to similar sections being swapped over. These sections contain the same genes but often different alleles
• this produces a new combination of alleles on the chromatids
• the chiasmata remain in place during metaphase and hold the homologues together on the spindle
• holding the homologous pairs on the spindle equator ensures that when segregation occurs, one member of each pair goes to each pole.