Lecture 10 : Meiosis

Question 1

Definition: Homologous chromosome

Answer 1

• Pair of chromosomes that are alike and come together in synopsis during prophase of the first meiotic division. They are NOT identical.

Question 2

What is the role of synapsis & how does it cause genetic variation?

Answer 2

Synapsis is the fusion of chromosome pairs during meiosis.

• the 2 homologous chromosomes (tetrads) then cross over causing exchange between non-sister chromatids. ( paternal and maternal chromatids )
• This interaction is called Chiasmata

Question 3

What occurs during the interphase stage?

Answer 3

• chromosomes replicate in S phase, but remain uncondensed.
• each chromosome contains IDENTICAL sister chromatids, connected at the centromere.
• centrosome replicates, forming 2

Question 4

What occurs during prophase I:

Answer 4

• longest phase (90%)
• chromosomes condense (x shape)
• Homologous chromosomes loosely pair along their lengths, aligned gene by gene ( NO COVALENT BONDS )
• each tetrad contains at least one chiasmata holding the homologues together until anaphase l
• centrosomes move to opposite poles & spindles form
• nuclear envelope breaks down
• late prophase l : kinetochore of each homologue attach to microtubles from one of the poles and begin to move towards metaphase plate.

Question 5

What occurs in Metaphase l :

Answer 5

• Pairs of homologous chromosomes (as tetrads) are located horizontally in the metaphase plate
• both chromatids of a homologue are attached to kinetochore microtubules. The other homologue likewise to the opposite pole.

Question 6

What occurs during Anaphase l :

Answer 6

• homologous chromosomes move toward opposite poles, guided by the spindle apparatus
• sister chromatids remain attached at the centromere and move as a single unit toward the same pole.

Question 7

What occurs during Telophase I?

Answer 7

• Each half of the cell has a complete haploid set of chromosomes
• BUT each chromosome is still composed of two sister chromatids, that are ALIKE and NOT IDENTICAL due to the chiasmata ( source of genetic variation )

Question 8

What occurs during the Cytokinesis stage?

Answer 8

• 2 haploid daughter cells are formed
• usually occurs simultaneously with telophase l
• Animals ; cleavage furrow
• Plants ; cell plate formation
• in some species the chromosomes decondense and the nuclear envelope and nucleoli re-form

Question 9

What is Interkinesis?

Answer 9

• The time between meiosis l and meiosis ll

- No chromosome replication occurs, only cell resting

Question 10

What is the role of Meiosis ll?

Answer 10

• the separation of sister chromatids

Question 11

What occurs during prophase ll?

Answer 11

• formation of spindle apparatus

- chromosomes move towards metaphase plate ll.

Question 12

What occurs during Metaphase ll?

Answer 12

• chromosomes are positioned on the metaphase plate
• due to the crossover, the chromatids of each chromosome are NOT identical
• each chromatid is attached to a Kinetochore

Question 13

What occurs during anaphase ll?

Answer 13

• separation of sister chromatids as two individual chromosomes to opposite poles
• non-kinetochore microtubules begin to lengthen, cell expands twice its size

Question 14

What occurs during Telophase ll and cytokinesis?

Answer 14

• Nuclei forms
• Decondensing of chromosomes
• one parent cell produces 4 genetically distinct daughter cells (haploid)
• each daughter cell is a gamete (23 chromosomes)

Question 15

How does Asexual reproduction differ?

Answer 15

• a single individual is the sole parent and passes all genes. The genome of offsprings are virtually exact (clones)
• binary fission, mitosis, budding
• in sexual reproduction, two parents cause unique combination of genes (genetic variation)