MEIOSIS Flashcards
What are homologous chromosomes pairs
Chromosomes that have the same “address” of alleel - these chromosomes are usually similar in size shape and genetic composition
What is diploid and haploid
Diploid is 2n =46
Haploid is n = 23
What is a karyotype
The complete diploid set of chromosomes arranged according to their size shape and number in homologous pairs - consists of 22 pairs of autosomes + 1 pair of gonosomes (XX or XY)
What 2 categories is the cell cycle divided into
- interphase
- dividing phase
What is interphase
- period between 2 consecutive cell divisions. Cell growth and replication of DNA take place during interphase. Note the chromatin network is visible during this phase
What are the 2 types of cell division
- mitosis
- meiosis
What is mitosis
- the division of somatic (body cells) for growth, repair damage of tissue and asexual reproduction
- 2 identical daughter cells produced from mother cell - they are identical to each other
Briefly describe the 4 phases of mitosis
P: -chromotain network shortens and thickens to become visible chromosomes (joined by centromere)
- nuclear membrane disappears
- centrioles move to opposite poles and spindle fibres develope
M: - chromosomes move to equator
- spindle fibres attach to centromeres
A: - chromotids separate and move to poles (now called daughter chromosomes)
- cytokinesis begins
T: - daughter chromosomes position themselves at poles
- spindle fibres disappear
- nuclear membrane is formed around daughter chromosomes + a nucleolus
- cytokinesis is now complete and 2 identical cells are complete
What is meiosis
Formation of gamete’s or spores - the chromosome number is halved (2n - n) it’s also known as reduction division
Why is there 2 stages of meiosis
- the first division the chromosome number is halved and genetic material is exchanged
- second division is like normal mitosis where daughter chromosomes separate from each other
What happens in prophase 1
- chromatin network shortens and thickens and becomes visible chromosomes
- homologous chromosomes arrange in pairs - they lie close together and from bivalent (2 homologous chromosomes/4 chromatids)
- each chromosomes of the bivalent splits into chromatids
- crossing over takes place - the place where crossing over happens /breakage occurs is the chiasmata
- the nucleus and nuclear membrane disappear
- Centrioles move to poles and spindle fibres form
What happens at metaphase 1
- bivalents position themselves on equator
- chromosomes lie in double row or either side of equator
- spindle fibres attach to centromeres
What happens in anaphase 1
- spindle fibres contract and shorten pulling chromosomes to poles
- homologous pairs are separated
- cytokinesis begins
What happens in telophase 1
- chromosomes group at poles
- spindle fibres disappear
- nuclear membrane and nucleolus forms
- cytokinesis is complete and there’s now 2 daughter cells - each daughter cell has haploid number (n) and the chromosomes are double stranded with exchanged genetic material
What happens in prophase 2
Nucleolus and nuclear membrane disappears + spindle fibres for,
What happens in metaphase 2
- chromosomes move to equator and align in a row
- spindle fibres attach to centromeres
What happens in anaphase 2
- centromere of each chromosome splits
- chromatids separate and move to opposite poles
- they are now daughter chromosomes
- cytokinesis begins
What happens in telophase 2
- the chromosomes in each daughter cell are single stranded with recombinant chromatids
-the spindle fibres disappear - nuclear membrane and nucleolus forms and cytoplasm divides
- cytokinesis is now complete and 4 daughter cells are formed each with haploid number
(4 sperm or 1 egg with 3 polar bodies that dissolve)
Purpose of meiosis
- formation of gametes(gametogenesis x oogenesis or spermatogenesis)
- takes place in ovaries or testes
What is the importance of meiosis
Chromosome number is halved
Gametes with different gene combinations are produced leads to genetic variation - this is because of crossing over
How is there genetic variation in meiosis
- crossing over - recombinant genes occur
- random assortment of chromosomes on the equator during metaphase 1
- random fertilisation - which sperm fuses with which egg - consequently the new individual is unique
What is aneuploidy
- a chromosome mutation where there is an extra chromosome or missing chromosome
An eg would be Down syndrome where there are 47 instead of 46 - the pair 21 doesn’t separate during anaphase 1
How are mitosis and meiosis similar list 3
- both a cell division process
- both require DNA replication during interphase before division begins
- both have the4 phases PMAT
What are the differences of mitosis and meiosis table form
Mitosis. | meiosis
- takes place in body cells - takes place in reproductive organs
- takes place in damaged. - takes place in sporangia of the sporophyte generation
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