http://www.archive.org/details/GeneticsLecture2831
- • Cell cycle is genetically regulated. WHY?
- o Too many cells = not enough food
- o Avoid mutations
- o Avoid cancer
- • Cell is built in with 3 checkpoints to control growth
- o G1 S Checkpoint → occurs at the end of G1 phase (prior to entering into S phase). Integrity of the DNA . . . are there any errors in the DNA. Also check the size of the cell.
- o G2 M Checkpoint → occurs at the end of G2 phase. Check two things:
- • Integrity of DNA – no mutations occurred during the previous round of division
- • Environmental conditions – can the environment support the cells
- o M Checkpoint → occurs early in Mitosis and ensures that the mitotic spindle has attached to the DNA correctly. If it is not attached correctly then things may come apart incorrectly and things such as down syndrome occurs.
- • Meiosis
- o Occurs in diploid (us) organisms. Functions to create haploid gametes, they have half of the chromosomal content.
- o Meiosis is a two step process.
- • At the beginning of meiosis I you have 4 copies of each chromosome
- • At the end of meiosis I you have two cells each of which have two copies of each chromosome.
- • At the end of Meiosis II you have 4 cells each of which have one copy of each chromosome
- o MEIOSIS I - Stages
- • Prophase I: genetic recombination occurring. In genetic recombination you take your parental chromosomes and you exchange information between one maternal chromosome (from Mom) and one paternal chromosome. End result is the production of a new chromosome that is a unique DNA sequence. Facilitates genetic diversity.
- - --5 subdivisions-- -
- Leptonema → chromatin condenses
- Zygonema → your homologous chromosomes pair with each other.
- Pachynema → chromosomes condense further. Process of synapsis → chromosomes in a homologous pair are intimately linked.
- Diplonema → tetrads appearing. Two pairs of homologous chromosomes come together to make a group of 4 chromosomes. IN this process genetic exchange occurs . . . essentially a different arrangement.
- Diakinesis → chromosome pairs begin to separate and exchange is completed. The important end result is 4 unique copies of the chromosome. (When you started you only had two)
- • Prometaphase I
- Like prometaphase in mitosis. (see 8/27 notes)
- When you get to meiosis II you start with a diploid cell. End point is two haploid cells per diploid cell.
- This is the process of meiosis as it is actually done in the body
- o Oogenisis → process of egg production in females. (slide 7, chap 2)
- • Begin with the oogonium, it has 4 copies of each chromosome. The oogonoium grows through a period of growth and maturation to produce the primary oocyte. It is the primary oocyte that enters meosis I. (The oogonium will not begin division until it has matured enough to become a primary oocyte.) Divide the primary oocyte in two.
- Secondary oocyte (diploid cell).
- First polar body → will not become an egg (endpoint)
- • This happens during oogenisis because during the physical division of cells in meiosis (oogenisis) the DNA is divided in two. When you get to cytokinesis you have an eneven cleavage of the cytoplasm. The primary oocyte gets the majority of the cytoplasm. The first polar body gets a small amount of cytoplasm. This occurs because the egg is the major donor to the cytoplasm of the zygote.
- • Another division occurs and the secondary oocyte splits into the ootid and second polar body. The ootid differentiates into the ovum which is a fully mature gamete.
- • HOW THIS ALL WORKS:
- In a human female meiosis I occurs during the embryonic period. A newborn girl has all of the secondary oocytes that she will ever produce.
- Meiosis II occurs during the menstrual cycle. Meiosis II only completes when fertilazation occurs.
- o Spermatogenesis - production of sperm in males (slide 7, chap 2)
- • Spermatogenium → undergoes growth and differentiation.
- • Primary spermatocyte then has 4 copies of each chromosome. Go through meiosis I and you produce two secondary spermatocyts. The sperm is not responsible for cytoplasmic delivery you can produce two secondary spermatocytes, they are diploids.
- • Then there is meiorsis II and there are 4 spermatids, haploid. Next is growth differentiation. Result is 4 functional spermatozoa.
- • In the human male this is an ongoing process after the onset of puberty.
- • SIDE NOTES: During meiosis you can have nondisjuntion events. This means that during anaphase when your chromosome pairs should split they fail to separate. This, in turn, can lead to a gamete with either too few or too many chromosomes. If these gametes are fertilized the end result is a trisomy on a monosomy.
- • Meiosis is critical for sexual reproduction and genetic diversity. It produces the necessary haploid gametes. The genetic recombination leads to an increase in diversity. The average person can produce 8 million unique gametes. Multiplied by the 8 million the other person has you will get 64x10^13 unique offspring.
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