II. Connections
1. Homologous chromosome & Duplicated chromosome
At the beginning of a cell's life cycle, a cell has 23 pairs of homologous chromosomes. A homologous chromosome is one chromosome in a pair that have the same genes, but one version given from the mom and the other version given from the dad. A duplicated chromosome is a pair of chromosomes that are completely identical to each other. A cell has duplicated chromosomes after S-phase, when all the DNA in a cell is copied.
2. Kinetochore & Microtubule Organizing Center (MTOC)
In an animal cell, the MTOC is a centrosome. This is the place where microtubule spindle fibers develop and grow. They connect to the kinetochores attached to the centromeres of the chromosomes during the prophase stage. The spindle fibers shorten starting from the centrosomes, pulling the chromosomes apart.
3. Haploid & Somatic
A somatic cell is any cell in the body that is not a germ cell. The only cells in the body that are haploid are gametes, or reproductive cells. Haploid cells only have half of the DNA a normal cell, or diploid cell, has. This is because the reproductive cells from the mom and from the dad will come together during reproduction and fertilize a zygote, a diploid cell.
4. Nucleosome & Dehydration Reaction
A nucleosome is a small stretch of DNA wound twice around a spool of proteins called a histone. Proteins are made up of amino acids linked together by peptide bonds. They form in a dehydration reaction because the OH molecule on the carboxyl group and the H atom in the amino group bond together forming water, which is lost during this processs
III. Few Essentials
1. Chromosomes during Meiosis
A. Prophase 1: chromosomes are in thread-like form; homologous pairs are together; swap segments
B. Metaphase 1: homologous chromosomes line up next to each other along the midway line of the cell
C. Anaphase 1: spindle fibers pull apart homologous chromosomes
D. Telophase 1: homologous chromosomes are now in different ends of the cell; all chromosomes are still duplicated
E. Prophase 2: no new DNA replication has occurred; spindle fibers attach to each chromosome
F. Metaphase 2: all chromosomes line up along the midway of the cell
G. Anaphase 2: attachment between sister chromatids of each chromosome breaks; one of each type of chromosome is moving towards the ends of the cell
H. Telophase 2: 4 cells result with a haploid number of chromosomes
2. Outline 10.4
I. Crossing Over in Prophase 1
A. All chromosomes in a germ cell condense in the same way
1. drawn close to its homologue
2. chromatids of 1 become stitched to chromatids of another
a. favors crossing over
3. 2 "nonsister" chromatids swap genes
B. Genes come in different forms- alleles
1. crossing over: chance to swap slightly different versions of information on gene products
2. crossing over leads to recombinations among genes of homologous chromosomes, and eventually to variation in traits among offspring
II. Metaphase 1 Alignments
A. major shufflings of chromosomes: transition from prophase 1 to metaphase 1
1. no particular pattern to the metaphase 1 positions of chromosomes
2. either homologous partner can end up at either spindle pole
3. 8,388,608 possible combinations of chromosomes
3. Similarities & Differences
Similarities
- starting cell is a diploid cell
- all chromosomes are duplicated from interphase
- both use spindles to sort and move chromosomes
Differences
- mitosis ends with 2 diploid cells; meiosis ends with 4 haploid cells
- mitosis happens in any part of the body; meiosis only occurs in the reproductive organs
- mitosis: basis of asexual reproduction and growth and tissue repair; meiosis: required step before the formation of gametes or sexual spores
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