Week 2 Part 1 Flashcards
Fertilized egg (totipotent stem cell)
Form from germline cells by specific type of cell division called meiosis - happens only in gonads and ovaries.- and only in gametes.
Halves the number of chromosomes in the final gamete cells
Somatic cells are diploid = 46 chromosomes in the final gamete cells
1 of each chromosome from mom and dad
Gametes are haploids = 23 chromosomes , 1 of each chromosome
Mitosis
One division
Two daughter cell per cycle
Daughter cells genetically identical
Chromosome number of daughter cells same as that of parent cell
Occur throughout life cycle
Used for growth, repair and asexual reproduction
Occurs only is somatic cells
- one replication of DNA followed by 1 cell division
Meiosis -
only in Germ line cells - diploid
Gamete = sperm and oocyte - haploid
-
-One replication of DNA followed by 2 rounds of cell division = 4 daughter cell each with with one copy of each chromosome.
Meiosis
Two divisions
Four daughter cell per cycle
Daughter cells are genetically different
Chromosome number of daughter cell half that of parent cell
In humans, completes after sexual maturity
Used for sexual reproduction, producing new gene combinations
Occurs in germline cells
Halves number of chromosomes
Adds genetic variation - paternal and maternal randomly divided between final gametes - in different cells
Meiosis 1 - reduction division - haploid -
Divides the homologous chromosomes
Maternal chromosome 1 would be in a different cell than paternal chromosome 1
Meiosis 2= equilateral division
Divides the replicated chromosomes into different cells
The two sister chromatids for chromosome 1 would be in different cells.
After meiosis 1 cell is haploid, paternal is in different cell than maternal chromosome. Meiosis two split exact copies 4 cells all haploid.
Meiosis 1 Begins when
cells enter prophase 1 from interphase
DNA was replicated during S phase of interphase
Prophase (Early)
Homologous begin to pair up
A synapsis forms - homologs are lined up gene for gene and held together by mixture of proteins and RNA
Crossing over occurs - only happens in meiosis 1
Segments of chromosomes can be exchanged between homologs.
Prophase (late)
Similar process as prophase of mitosis
Spindle begins to form
Nuclear envelope breaks down
Chromosomes condense
Metaphase
Homologs are aligned by spindle in middle of cell
Anaphase
Homologous chromosomes separate to poles of cell Sister chromatids (replicated chromosomes ) are still attached at centromere and segregate together.
Telophase
Nuclear envelope begins to reform
Spindle begins to break down
End result of meiosis is two haploid daughter cells
Different mitosis- anaphase - sister chromatids are pulled apart, meiosis 1. Sister chromatids are still held together in same cell, its the homologous chromosomes that are pulled apart. Result if two haploid daughter cells
Meisisis 2 differences
Different of cell division, before meiosis 2 DNA is not replicated again.
Meiosis 2
After telophase 1, cells enter another interphase , but DNA is not replicated
Crossing over only occurs in prophase 1, not in prophase 2
At anaphase 2, sister chromatids are pulled apart.
Final result of meiosis is four daughter cell with one copy of each chromosome
23 total chromosomes
Meiosis 1 vs Meiosis 2
-daughter cells that have 1 pair of each chromosomes.
Meiosis 1 - separate homologous pairs of chromosomes, sister chromatids still attached. - 2 daughter cell haploid 1 of each of each chromosome.
- sister chromatids pull apart. - 4 cells each has 1 chromosome- 1 of the sister chromatids from s phase. - not generically identical from where it came from.
Independent assortment
Alignment of homologous pairs in metaphase is random
Each daughter cell is a mix of paternal and maternal chromosomes
More than 8 million combinations
When two gametes combine the total number of combinations is in the trillions
Crossing over
Makes the possible combination limitless cuz each chromosome is a mixture of maternal and paternal DNA - Genetic variation - good.
Spermatogenesis
continual process that begins at puberty in males
Begins with spermatogonium
Diploid cell that divides mitotically.
Is a stem cell
One daughter cell will remain a stem cell
One daughter cell will develop into a spermatocyte
Primary spermatocyte
replicates DNA then enters meiosis1
After meiosis 1, primarily spermatocyte become secondary spermatocyte
After meiosis 2, cells become spermatids which become sperm.
Meiosis 1 occurs when primary spermatocyte become secondary. Meiosis 2 occurs when secondary spermatocyte become spermatids.
Oogenesis
- process that begins during fetal development in females
- Begins with oogonium. Cell grows and replicated DNA becoming oocyte
Primary oocyte undergoes meiosis 1
Produces a large secondary oocyte and a much small first polar body ( unequal cell division)
At birth females have -1 million oocytes arrested in prophase 1
Ovulation
After puberty one or more oocytes resume meiosis during each menstrual cycle
EGG
After puberty one or more oocytes resume meiosis during each menstrual cycle
