Lecture 8a Flashcards
When does mitosis and meiosis happen for plants and animals?
Mitosis happens:
After fertilization
During growth & development
To replace dead or damaged cells: stem cells divide (stem cells in animals found throughout the body; plants have stem cells in limited areas)
Meiosis happens
only in gonads in animals &
in specialized structures in plants
Cell division & chromosome number
Mitosis: chromosome number of daughter cells same as parent cell
Meiosis: chromosome number of daughter cells ½ the number same as parent cell
2 categories of chromosomes: autosomes & sex chromosomes
In humans: autosomes: # 1 to 22 (chromosomes are numbered based on size, shape, genes present)
Sex chromosomes: 23rd chromosome: X or Y
Karyotype
Karyotype: profile of a person’s chromosomes
How many sets of chromosomes do gametes and body cells have?
Gametes have 1 set (1n) of chromosomes; body cells have 2 sets (2n) - diploid cells Gametes 23 chromosomes (1n) 22 autosomes + 1 sex chromosome
Fertilized egg & Body cells
46 chromosomes (2n):
44 autosomes +
2 sex chromosomes
What is a set of chromosomes?
Chromosomes come in matched sets known as homologous pairs.
The 46 chromosomes of a human cell are organized into 23 pairs, and the two members of each pair are said to be homologues of one another (with the slight exception of the X and Y chromosomes, see below).
Alleles:
alternate versions of genes on homologous chromosomes
2 alleles:
1 says curly hair: GGGCATGCCAATT
1 says straight hair: ATGCATGCCAATT
Homologous pair of chromosomes:
each with genes for the same characters. One inherited from mom, one inherited from dad.
Gene:
: A segment of DNA at a specific locus on a specific chromosome. It contains the info for making an RNA: if mRNA, then it codes for a particular protein (ex D codes for lactase)
A pair of alleles.
Alleles: alternate versions of the same gene pair. The alleles can code for the same version of the trait (both code for a long nose) or different versions (one codes for a long nose, the other for a short one)
Sex chromosomes: 2 copies for each allele?…it depends
Females have 2 copies of all 800 genes on the X-chromosome.
775 in the X-specific region + 25 in the homologous region
Males have 2 copies of the 25 genes in the homologous region of the y-chromosome.
1 copy of the 775 genes in the X-specific region
1 copy of 55 genes in the
y-specific (differential) region
slide 17
Fertilized egg & all somatic cells have …. set(s) of chromosomes
Fertilized egg & all somatic cells have 2 sets of chromosomes
23 chromosomes in egg (1-22 + X) +
23 chromosomes in sperm (1-22 + X or Y)
After fertilization you (zygote) have 46 chromosomes:
44 autosomes + 2 sex chromosomes
What do stem cells do?
Stem cells in the gonads:
1. Stem cells that maintain the stem cell population go through the cell cycle that includes mitosis. 1 cell → 2 → 4 → …
- In response to hormones, some of the stem cells go through the cycle that includes meiosis. These germ cells will become gametes: they are haploid cells that have left the cell cycle.
Stem cells in the rest of the body
1. Stem cells that maintain the stem cell population: go through the cell cycle that includes mitosis. 1 cell → 2 → 4 → …
- Stem cells that receive a chemical signal that causes them to become a differentiated cell leave the cell cycle. → bone cells, neurons,…
Cell cycle or not
Cells in cell cycle: stem cells Interphase: G1,S,G2 Cell division (mitosis) Cytokinesis Back to interphase If it remains a stem cell (no differentiation occurs), then it may go through mitosis & cytokinesis again
When cells become differentiated they leave the cell cycle
Cells are in Go
Performing their job (ex making hormones)
Make new organelles as needed
For stem cells throughout the body & in the gonads: Cell cycle
interphase, mitosis, & cytokinesis
Production of gametes cell cycle
Cell Cycle: interphase, meiosis, & cytokinesis (only in the gonads)
Interphase
Interphase is exactly the same, whether cell will go through mitosis or meiosis
The cell is preparing for cell division
3 PARTS:
G1 (Gap 1): Most organelles are replicated.
S phase: copy all the chromosomes. Synthesize (make) DNA & the associated proteins
G2 (Gap 2): centrioles (needed in cell division) duplicated + make other proteins that will be needed
Go
When a cell differentiates (ex becomes a muscle cell), it stops dividing, so it leaves the cell cycle.
It’s said to be in Go
Interphase: S phase
S= synthesis phase
Chromosomes duplicated: each now consists of 2 identical sister chromatids Chromosomes duplicated (aka replicated): each chromosome now consists of 2 identical sister chromatids, connected at their centromeres.
Connected at their centromeres
Chromatid vs chromosome
1 replicated chromosome made up of 2 sister chromatids
Each chromatid made up of 1 double-stranded DNA molecule and its associated proteins
When 2 sister chromatids separate, each chromatid now called a chromosome: have now 2 unreplicated chromosomes.
How are the sister chromatids held together?
Cohesion (a complex of many proteins)
Centromere:
Highly condensed region of DNA
At anaphase, an enzyme cleaves off much of the cohesion complex, allowing the sister chromatids to be separated
