Cell Cycle and Cell Division

CST Review PowerPoint

- Somatic (Body) cells are produced through mitosis while gametes (sex cells) are produced through meiosis.

- Mitosis conserves chromosome number while meiosis produce daughter cells with half the chromosome number of the parent cell.

- Mitosis involves one division while meiosis involves two divisions (Meiosis I and Meiosis II).

Cell Division

-Chromosomes occur in pairs in somatic cells.

-When a cell has the full set of chromosomes, it is described as diploid (2n), and when it has half, it is described as haploid (n).

- Egg and sperm cells (pollen grains as well) are haploid cells.

Chromosomes

More about chromosomes…

-This is a duplicated chromosome.

-The chromosome is a complex of DNA and protein.

-The centromere (not always centrally-located) holds the sister chromatids together.

Chromosomes are only visible as discrete

structures during cell division.

The Cell Cycle

-A cell goes through a cycle of growth and division.

-The life cycle of a cell is called the cell cycle.

Cell Cycle Stages

G1 phase. Metabolic changes prepare the cell for division. At a certain point - the restriction point - the cell is committed to division and moves into the S phase.

S phase. DNA synthesis replicates the genetic material. Each chromosome now consists of two sister chromatids.

G2 phase. Metabolic changes assemble the cytoplasmic materials necessary for mitosis and cytokinesis.

M phase. A nuclear division (mitosis) followed by a cell division (cytokinesis).

The period between mitotic divisions - that is, G1, S and G2 - is known as interphase.

Mitosis -The parent cell that is

prepared for mitosis has duplicated chromosomes.

-At prophase, the nuclear membrane breaks down, while the chromosomes shorten and thicken). The other structures important for mitosis are also forming (i.e. the centrioles).

Mitosis, continued…

Metaphase- The mitotic spindle apparatus has now formed. The chromosomes are lined up along the cell's equator, also known as the equatorial plate, and are attached to the mitotic spindle apparatus via microtubules.

Anaphase - The newly formed chromosomes are pulled toward opposite poles of the cell.

Telophase

The chromosome have finished their migration to the poles and the mitotic structures breakdown.

The plasma membrane of the cell pinches down along the equator creating two separate cells (similar to twisting a long balloon and forcing it to pinch in the middle).

At this time, the chromosomes become indistinct (as they are during Interphase), the nuclear membrane forms again and the nucleolus reappears.

In plant cells, ‘pinching’ does not occur; a cell plate forms instead. Cell Plate

Cleavage Furrow

I Prefer Milk and Tea!

Interphase Prophase

Metaphase

Anaphase Telophase

Meiosis

Meiosis is the type of cell division by which sex cells (eggs and sperm) are produced. Meiosis involves a reduction in the amount of genetic material.

Meiosis comprises two successive nuclear divisions with only one round of DNA replication.

Meiosis I is when a diploid cell produces two haploid cells.

Meiosis II involves separation of chromatids and the production of four haploid cells. Meiosis II is just like mitosis.

Homologous Chromosomes

In diploid cells, chromosomes can be arranged in pairs.

Each homologous pair is similar in size and gene locations.

The sex chromosomes are an exception.

Crossing-over during synapsis (Prophase I) is important because it gives rise to greater genetic variation in the

sex cells formed.

During tetrad formation, homologous chromosomes may swap genes.

Crossing over produces chromosomes with new combinations of genes—and offspring, called recombinants, with new combinations of traits not seen in either parent. Generally the closer two genes are on a chromosome, the less likely they are to be separated by crossing over.

Synapsis and crossing over occur only in meiosis.

Another Look: Crossing Over

At the end of Meiosis II, each of the four haploid cells contain half the number of chromosomes present in the parent cell. These chromosomes

are unduplicated.

-Egg formation happens inside ovaries.

-Primary oocytes (2n) divide through Meiosis I to form haploid secondary oocytes.

-Secondary oocytes undergo Meiosis II during fertilization.

-Cytokinesis is unequal in both Meiosis I and II, leading to three very small polar bodies, and one large ovum.

Oogenesis

Polar bodies degenerate.

-is sperm production.-occurs inside the male

testes.-involves meiosis.-Starting at puberty, a

male will produce literally millions of sperm every single day for the rest of his life.

Spermatogenesis