Steps of Mitosis
Goal of Mitosis: two daughter cells inherit an equal and identical complement of chromosomes |
3 things that Mitosis does: 1.) development 2.) growth cell replacement 3.) asexual reproduction |
Interphase: Period when cell isn't in non dividing state; consists of G1, S-phase, G2 |
S-Phase: DNA replication |
G1 Phase: the cell grows physically and increases the volume of both protein and organelles. |
G2 Phase: Organelles and proteins for cell division are produced; Getting ready for Mitosis |
Prophase: Condensation of chromosomes,the movement of the centrosomes, the formation of the mitotic spindle, and the beginning of nucleoli break down |
Metaphase: Duplicated chromosomes move to the middle, spindle fibers connect to centromeres |
Anaphase: Sister chromatids separate from one another and are pulled into opposite poles of the cell |
Telophase: Chromosomes begin to decondense, spindle breaks down, nuclear membrane and nucleoli re-form (nuclear envelope) |
Cytokinesis: splits the parent cell into two identical daughter cells |
Oogenesis
diploid oogonium go through mitosis until one develops into a primary oocyte, which will begin the first meiotic division, but then arrest (in prophase 1) ; it will finish this division as it develops in the follicle, giving rise to a haploid secondary oocyte and a smaller polar body
- Meiosis starts in fetus
Oogenesis -> 1 haploid, ovum + 2-3 polar bodies |
Arrest in Oogensis
• At birth, Primary oocyte, Arrest Prophase I
• Prophase I
arrest in fetal
development
• Continue at
puberty, Completes meiosis I, Secondary oocyte, Continue meiosis II, Arrest Metaphase II
• Continue and
complete Meiosis
II only upon
fertilization |
Spermatogenesis
The origin and development of the sperm cells within the male reproductive organs, the testes.
- diploid spermatogonium, produces two diploid intermediate cells called primary spermatocytes.
- Haploid sperm, only the egg and sperm cells are haploid.
Spermatogenesis -> 4 haploid sperm |
Polar Bodies
- Asymmetric cell divisions in Meiosis I and II
- Asymmetric partitioning of cellular material
- After 2 rounds of meiosis, only 1 haploid ovum vs. 4 as with sperm
A polar body is the byproduct of an oocyte meiotic division |
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Comparison of Mitosis and Meiosis
Mutations with Meiosis
• Inherited mutations are present in the parents, inherited from their parents, and can be passed to gametes |
• But NEW spontaneous mutations can also occur in germline cells in meiosis |
• New mutations in gametes more likely to develop or accumulate with age |
• Oocytes more often have chromosomal imbalances |
• Missing or extra entire chromosomes, usually due to non-disjunction |
• Sperm more often have dominant single gene mutations |
• “Paternal age effect” |
Vocabulary
Sister Chromatid = identical copies formed by the DNA replication of a chromosome, with both copies joined together by a common centromere. In Mitosis |
Homologs = Two chromosomes in a pair – normally one inherited from the mother and one from the father. In Meiosis |
Haploid = the presence of a single set of chromosomes in an organism's cells (1n) |
Diploid = A cell containing two copies of each chromosome (2n) |
Somatic Cell = any cell of a living organism other than the reproductive cells |
Germline Cell = The cells that form eggs in females and sperm in males |
Allele = variation of the same sequence of nucleotides at the same place on a long DNA molecule |
Crossing Over = When two chromosomes — one from the mother and one from the father — line up, parts of the chromosome can be switched |
Ploidy = number of complete sets of chromosomes in a cell |
Reductional division = The first cell division in meiosis, the process by which germ cells are formed, MEIOSIS |
Equational division = the process of cell division wherein the chromosomes replicate and get equally distributed into two daughter cells, MITOSIS |
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Steps of Meiosis
Meiosis = Reductional division |
Goal of Meiosis: produce gametes, the sperm and eggs, with half of the genetic complement of the parent cells. |
Prophase 1: replicated chromosomes condense, homologous chromsomes pair up, crossing over occurs between homologous chromosomes, the spindle is formed, and the nuclear envelope breaks apart into vesicles |
Metaphase 1: chromosomes condense and move together, aligning in the center of the dividing cell. |
Anaphase 1: Homologous chromosomes are pulled to opposite poles of the cell but sister chromatids remain joined at their centromeres |
Telophase 1: the chromosomes are enclosed in nuclei, four haploid daughter cells, generates genetic diversity |
Meiosis 2
Prophase 2: chromosomes condense, and a new set of spindle fibers forms. NO crossing over |
Metaphase 2: INDIVIDUAL centromeres of the paired chromatids align along the equatorial plate in both cells |
Anaphase 2: Sister chromatids SEGREGATE from each other and migrate to opposite ends of the cell |
Telophase 2: cell division begins again in each of the two daughter cells, creating 4 daughter cells |
Mitosis = Equational division |
Difference of Meiosis 1 and Meiosis 2
The goal of meiosis I is to separate homologous chromosomes. The goal of meiosis II is to separate sister chromatids |
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