Week 11 - Reproduction: meiosis

Question 1

Meiosis

Answer 1

Producing haploid cells

Question 2

Fertilisation

Answer 2

• Haploid cells producing diploid cells.

- Union of gametes (sperm and egg)

Question 3

Gametes

Answer 3

• Sex cells in animals (sperm and eggs), arise from germ cells.
• Only cells in the human body produce by meiosis

Question 4

Zygote

Answer 4

• a fertilised set and has one set of chromosomes from
  each parent.
• produces SOMATIC CELLS by mitosis and develops into an adult

Question 5

Diploid cells

Answer 5

Somatic (non reproductive) cells of adults have 2 sets of chromosomes

Question 6

Haploid cells

Answer 6

Gametes (eggs and sperm) - only 1 set of chromosomes

Question 7

Karyotype

Answer 7

Particular array of chromosomes in an organism arranged according to size, staining properties and location of centrometre
Human cell - 23 pairs
→XX= female, XY=male

Question 8

Prophase I

Answer 8

• Chromosomes coil tighter and become visible, nuclear envelope disappears, spindle forms
• Each chromosomes has 2 sister chromatids

Question 9

Features of Meiosis - Synapsis

Answer 9

• During early prophase I
• Homologous chromosomes become closely associated or ‘become paired’
• Include formation of synaptonemal complexes → formation of tetrad or bivalents
  →meiosis specific from of cohesion

Question 10

Crossing over

Answer 10

• Genetic RECOMBINATION between nonsister chromatids
• Allows homologues to exchange chromosomal material
• Alleles of genes that were formerly on separate homologues can be found on smae homologue
• CHIASMATA: site of crossing over → contact maintained until anaphase I

Question 11

Prophase I - continued

Answer 11

Synapsis

• homologues become closely associated
• crossing over occurs
• remain attached at chiasmata

Question 12

Metaphase I

Answer 12

Terminal chiasmata hold homologues together following crossing over

• microtubules from opposite poles attach to each homologue → not each sister chromatid
• HOMOLOGUES ARE ALIGNED AT THE METAPHASE PLATE SIDE BY SIDE
• Orientaiton of each pair of homologues on spindle is RANDOM

Question 13

Anaphase I

Answer 13

• microtubules of spindle shorten → chiasmata break
• HOMOLOGUES ARE SEPARATED FROM EACH OTHER AND MOVE TO OPPOSITE POLES → sister chromatids remain attached to eachother at their centromeres
• each pole has a complete haploid set of chromosomes consisting of one member of each homologous pair
• independent assortment of maternal & paternal chromosomes

Question 14

Telophase I

Answer 14

• NUCLEAR ENVELOPE RE-FORMS AROUND EACH DAUGHTER NUCLEUS
• sister chromatids are no longer identical because of crossing over (Prophase I)
• Cytokinesis may or may not occur
• Meiosis II occurs after an interval of variable length

Question 15

Meiosis I summary

Answer 15

REDUCTION DIVISION

• results in daughter cells that contain only one set of originial homologous pairs
• each chromosome in a daughter cell is derived from just one parent.
• Resuting cells retain both sister chromatids, so they are still diploids
• no DNA replication occurs before going into meiosis II
• Meiosis II will separate sister chromatids

Question 16

Meiosis II

Answer 16

• SEPARATION of sister chromatids into haploid daughter cells
• no DNA replicatin before this page
• resembles a mitotic division

Question 17

Prophase II

Answer 17

Nuclear envelope dissolve and new spindle apparatus forms

Question 18

Metaphase II

Answer 18

Chromosome align on the metaphase plate

Question 19

Anaphase II

Answer 19

Sister chromatids align on the metaphase plate

Question 20

Telophase II

Answer 20

Nuclear envelope re-forms around 4 sets of daughter chromosomes

Question 21

Mitosis

Answer 21

2 genetically IDENTICAL daughter cells

Question 22

Meiosis

Answer 22

4 genetically NON-IDENTICAL daughter cells

Question 23

Independent assortment

Answer 23

• each pair of chromosomes sorts maternal and paternal homologues into daughter cells independently of other pairs

Question 24

Crossing over

Answer 24

• Produces RECOMBINANT CHROMOSOMES, which combine genes inherited from each parents
• crossing over makes genes located far apart on same chromosome assort independently
• in crossing over, homologous portions of two nonsister chromatids trade places
• crossing over contributes to genetic variation into a single chromosome

Question 25

Random fertilisation

Answer 25

- random fertilisation adds to genetic variation because any sperm can fuse with any ovum (unfertilised egg) - fusoion of two gametes produces a zygote with any of about 70 trillion diploid combinations

Question 26

Aneuploidy- errors in meiosis

Answer 26

State of not having euploidy | - where there is more or less chromosomes other than multiples of a full set

Question 27

Nondisjunction

Answer 27

- Failure of chromosomes to move to opposite poles duringeither meiotic division

Question 28

Aneuploid gametes

Answer 28

Gametes with missing or extra chromosomes