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