Meiosis Flashcards
Purpose of meiosis
- To create sex cells (gametes) of the correct number of chromosomes. Takes a diploid cell and works to halve it
- To increase the genetic variation during sexual reproduction
Somatic
Body cells, diploids (2n)
Gametes
Sex cells, haploids (n)
What is “n” for humans
n = 23
Chromosome 23
Chromosome 23 determines the gender of the offspring.
- Females have two homologous X chromosomes (XX).
- Males have one X chromosome and one Y chromosome (XY), which are not homologous.
Diploid are made …
23 chromosome from the father and 23 from the mother, homologous chromosomes join together from 23 pairs and 46 total
What is the only non homologous chromosomes except
Male chromosome are the only exception when it comes to homologous pairs. Because in order to make a male it requires both an X and Y chromosome
Prophase 1
- Chromatins condense to form chromosomes, which contains 2 sister chromatids
- Sister chromatids are held together by a centromere in
- Homologous chromosome (4 chromatids), which consist of two sister chromatids join together where they exchange genetic information through cross over
Cross over
Homologous chromosomes join up with each other, where they exchange genetic information (segments) to increase the genetic variation
Metaphase 1
- Homologous chromosome pairs line up randomly along the middle
- Spindle fibers attach the the centromere of each chromosome and mover them
Anaphase 1
- Spindle fibers shorten pulling the homologous chromosomes away from each other and to opposite poles
Telophase/cytokinesis 1
*The nucleus and nuclear membrane begin to reform
* The cell begins pinching inward, as the cleavage furrow forms, making two new cells
* Begin to un condense slightly
Prophase 2
- each daughter will undergo prophase two,
*they are now haploids (23 chromosomes) - New centrioles in spindle fibres will form
- homologous chromosomes are no longer present only chromosomes which contain two sister chromatids,
*Chromosome will condense again - nucleus and nuclear membrane will disappear
Metaphase 2
Spindle fibers will attach to the centromere between the sister chromatids (chromosome) and line them up along the metaphase plate (middle) of the the cell
Anaphase 2
Spindle fibers shorten and separate the sister chromatids, pulling them away from each other to opposite poles
Chromosomes now consist of one sister chromatid
Telophase/ cytokinesis 2
The nucleus and nuclear membrane reform
Cell pinches inward as the cleavage furrow forms
The chromosome un condensed
Forms 4 new cell
Goal of meiosis 1
Separate homologous pairs, reducing the chromosomes number by half. and rise to two daughter cells with 23 chromosomes each which consist of 2 sister chromatids
Goal it meiosis 2
The goal is to separate sister chromatids and give rise to two more cells
Independent assortment and cross over genetic variation
Independent assortment is the random lining up of the homologous chromosomes during metaphase 1 and their separation during anaphase 1. Along with cross over that happens during prophase 1, which randomly exchanges genetic information with homologous chromosomes, they increase the genetic variation and uniqueness between each gamete
gametogenesis
Is the process in which a diploid cell is formed into a haploid cell by..
1) meiosis
2) cellular differentiation (spermatogenesis, oogenesis)
spermatogenesis
- Occurs in males testicles
- A spermatogonium undergoes mitosis. One of the daughter cell will stay a spermatogonium while the other will become a spermatocyte
- A spermatocyte will undergo meiosis and give rise to four genetically unique sperm cells
- About 1 billion sperm is produce in a day
Oogenesis
*Occurs in the female ovaries
* oogonium undergoes mitosis, producing a oogonium and a oocyte.
*Oocyte undergoes meiosis which gives rise to the ovum and the polar bodies
*the ovum receives most of the cytoplasm and will become the egg
*the polar bodies receive less cytoplasm and will eventually die and be absorbed
* Only one ovum is produced
Similarly between mitosis and meiosis
- Both undergo DNA replication during interphase stage S
- Both give rise to daughter cells
- Both undergo similar stages ( P M A T)
Differences between mitosis and meiosis
*Meiosis gives rise to 4 genetically unique daughter cells, while mitosis gives rise to two genetically identical daughter cells
*Meiosis is for genetic variation and sexually reproduction, while mitosis is for growth repair and and asexually reproduction
- meiosis gives rise to haploids (23 chromosomes), while mitosis gives rise to diploids (46 chromosomes)
