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Flashcards in Week 11 - Reproduction: meiosis Deck (28)
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1

Meiosis

Producing haploid cells

2

Fertilisation

-Haploid cells producing diploid cells.
-Union of gametes (sperm and egg)

3

Gametes

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

4

Zygote

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

5

Diploid cells

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

6

Haploid cells

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

7

Karyotype

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

8

Prophase I

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

9

Features of Meiosis - Synapsis

- 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

10

Crossing over

- 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

11

Prophase I - continued

Synapsis
- homologues become closely associated
- crossing over occurs
- remain attached at chiasmata

12

Metaphase I

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

13

Anaphase I

- 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

14

Telophase I

- 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

15

Meiosis I summary

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

16

Meiosis II

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

17

Prophase II

Nuclear envelope dissolve and new spindle apparatus forms

18

Metaphase II

Chromosome align on the metaphase plate

19

Anaphase II

Sister chromatids align on the metaphase plate

20

Telophase II

Nuclear envelope re-forms around 4 sets of daughter chromosomes

21

Mitosis

2 genetically IDENTICAL daughter cells

22

Meiosis

4 genetically NON-IDENTICAL daughter cells

23

Independent assortment

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

24

Crossing over

- 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

25

Random fertilisation

- 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

26

Aneuploidy- errors in meiosis

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

27

Nondisjunction

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

28

Aneuploid gametes

Gametes with missing or extra chromosomes