meiosis cellular reproduction of reproductive cells
TRANSCRIPT
MEIOSIS
Cellular Reproduction of Reproductive Cells
Vocab you should know… Meiosis: cell division where one body cell
produces four gametes, each containing half the number of chromosomes as a parent’s body cell
Homologous Chromosomes: paired chromosomes w/ genes for the same traits arranged in the same order
Gametogenesis: process by which male & female gametes are formed
Spermatogenesis: process by which male gametes form
More Vocab… Oogenesis: production, growth &
maturation of an egg (ovum) Zygote: diploid cell formed when a
sperm fertilizes an egg Crossing Over: exchange of genetic
material b/t nonsister chromatids from homologous chromosomes during prophase I of meiosis; results in new allele combinations
More Vocab… Genetic Recombination: major source of
genetic variation among organisms caused by reassortment or crossing over during meiosis
Nondisjunction: failure of homologous chromosomes to separate properly during meiosis; results in gametes with too many or too few chromosomes
Fertilization: fusion of male & female gametes Independent Assortment: random distribution
of the pairs of genes on different chromosomes to the gametes
More Vocab… Gamete: male & female sex cells; sperm &
eggs Tetrad: 4 chromatids in a pair of
homologous chromosomes that come together as a result of synapsis during meiosis
Polar Body: a short-lived product of the formation of gametes by meiosis
Gene: short segment of DNA containing instructions for a single trait
More Vocab…
Synapsis: the pairing of homologous chromosomes during meiosis
Homologue: something homologous Ovum: a mature egg cell (female
reproductive cell) Spermatid: immature sperm cell that
has almost completed development Spermatazoa: mature male
reproductive cell
Cell Cycle for Reproductive Cells
Interphase Meiosis I: prophase I, metaphase I,
anaphase I, telophase I Cytokinsis I / interkinesis Meiosis II: prophase II,
metaphaseII, anaphase II, telophase II Cytokinesis / Cytokinesis II
Cell Cycle of a Reproductive Cell Interphase – long first stage of cell grow &
DNA duplication (same as in mitosis) Meiosis I – similar to mitosis, results in
production of 2 2n (diploid) cells Interkinesis or Cytokinesis I – division of
cytoplasm to create 2 separate cells Meiosis II – similar to mitosis also, except
that 2 cells are undergoing it at the same time, resulting in 4 n (haploid) cells
Cytokinesis II – division of cytoplasm, similar to mitosis, except results in 4 cells
Interphase
Just as in mitosis, prior to division, the cell must duplicate all of its contents, including its DNA and must grow in size in order to be able to accommodate all of the duplicate material
Humans = 46 chromosomes, after interphase = 46 x 2 = 92
Meiosis I
There are four stages to meiosis IProphase IMetaphase IAnaphase ITelophase I
Prophase I DNA coils into chromatids Sister chromatids pair up Spindle fibers appear Nucleolus & nuclear membrane
disappear **synapsis occurs: each pair of sister
chromatids lines up next to its homologue forming a tetrad (the two pairs of sister chromatids join together instead of a pair of just a pair of sister chromatids)
Prophase I
Prophase I cont’d
In the tetrad, chromosomes are aligned lengthwise, so genes of one chromosomes are adjacent to the same genes on the other chromosome
Prophase I – crossing over During synapsis, chromosomes twist
around each other Sometimes they twist so tightly they
exchange genetic material = crossing over In humans this happens 2-3 times for every
pair of chromosomes Crossing over can result in new traits w/in
the organism = genetic recombination After prophase I = 92 chromosomes
Crossing Over
Metaphase I Tetrads line up randomly at the
equator Spindle fibers from the poles
(centrioles) attach to the centromere of each pair of the homologous chromosomes
After metaphase I = 92 chromosomes
Metaphase I
Metaphase I
Anaphase I Homologous chromosomes (sister
chromatids) move to the poles, each pair of the tetrad to the opposite pole
Independent assortment = the random separation of homologous chromosomes resulting in genetic variation (this is what is responsible for the differences in individuals)
After anaphase I = 92 chromosomes
Anaphase I
Independent Assortment
Telophase I
Homologous chromosomes reach the opposite poles
Spindle fibers break downNuclear envelopes forms around each
set of chromosomes, nuclei reappearAfter telophase I = 92 chromosomes,
46 in each new nucleus
Telophase I
Interkinesis or Cytokinesis I
Interkinesis, like cytokinesis, divides the cells cytoplasm resulting in 2 2n (diploid) cells, each having a complete set of DNA
Meiosis I starts w/ 1 2n cells, at the end of Meiosis I, we have 2 2n cells
Each new cell has 46 chromosomes
Interkinesis
Meiosis II After Meiosis I is complete, the two cells go
directly into Meiosis II ***No Interphase*** this means no
duplication of chromosomes Meiosis II has four phases
Prophase II Metaphase II Anaphase II Telophase II
Cytokinesis II – division of cytoplasm of both cells to create 4 separate cells
Prophase II
Nuclear membrane breaks down, nucleoli disappears
Spindle fibers form Chromatin condenses Sister chromatids pair up After prophase II = 46
chromosomes in each cell
Prophase II
Metaphase II
Pairs of sister chromatids move to equator in each cell
After metaphase II = 46 chromosomes in each cell
Metaphase II
Anaphase II
Chromatids separate & move towards opposite poles in each cell
After anaphase II = 46 chromosomes in each cell
Anaphase II
Telophase II
Nuclear membranes form around all 4 sets of chromosomes
Spindle fibers break down in all cells Nucleoli appear in the nucleus of all 4
cells Chromatids uncoil into chromatin After telophase II = 46 chromosomes
in each cell, 23 in each new nucleues
Telophase II
Cytokinesis II
Cleavage furrow deepens until it pinches off each cell, creating 4 new cells
All 4 cells are haploid (n) (contain 1 set of chromosomes)
After cytokinesis II = 23 chromosomes in each new cell
Cytokinesis II
Gametes & Gametogenesis
Meiosis results in the production of gametes
In humans: Females: oogenesis Males: spermatogenesis
Oogenesis Diploid reproductive cell divides meiotically
to form 1 mature ovum (egg) But…meiosis results in the formation of 4
cells…how is it that we only get 1 mature ovum?
Ova are very large, so when cytokinesis occurs, one cells get the majority of the cytoplasm, this becomes the ovum
the other 3 cells do not receive enough cytoplasm to survive, they are called polar bodies & will degenerate (disappear)
Oogenesis
Spermatogenesis
Diploid reproductive cell divides meiotically to form 4 haploid spermatids
Spermatids develop into mature spermatozoa
Spermatogenesis
Fertilization
The process of fusing a male and female gamete to produce offspring
Only occurs b/t two gametes, haploid (n) cells
Results in an offspring that has diploid (2n) cells (n x n = 2n)
Fertilization
Cancer
Uncontrolled cell division that may be caused by environmental factors &/or changes in enzyme production in the cell cycle
Basically, when the internal timing of a cell causes it to repeatedly divide at a faster than normal rate
Meiosis Quiz
1. What happens to the DNA of a reproductive cell during interphase?
2. Synapsis occurs during which phase of Meiosis (be sure to note I or II)?
3. What does synapsis result in?4. During what phase does crossing
over occur in?
Meiosis Quiz cont’d
5. What is crossing over?6. The random separation of
homologous chromosomes resulting in genetic variation is ____ _____?
7. Interkinesis/Cytokinesis I results in the formation of 2 ________ cells.
8. Cytokinesis II results in the formation of 4 _______ cells.
Meiosis Quiz Cont’d
9. Oogenesis results in the formation of _____ mature egg cells & 3 ______ ______.
10.Spermatogenesis results in the formation of ________ spermatids.
http://faculty.clintoncc.suny.edu/faculty/Michael.Gregory/files/bio%20101/Bio%20101%20Lectures/Meiosis/meiosis.htm