Cell Division

Lab 10

Cell Division

According to cell theory – cells only come from other living cells.

Information – encoded within DNA – must be replicated so each new cell has a copy.

Eukaryote Cell Cycle

DNA is contained in multiple chromosomes (a single long DNA molecule packaged with proteins into a compact shape).

Cell Division in Eukaryotes

Mitosis – cell division in somatic (nonreproductive) cells.

Meiosis – cell division in reproductive cells. Results in eggs or sperm.

The Cell Cycle

Interphase – G1, S, and G2 together. G1 phase – first growth phase S phase – synthesis phase, replication

of DNA. G2 phase – second growth phase,

replication of mitochondria, chromosomes condense.

M phase – mitosis C phase - cytokinesis

Chromosomes

Homologous chromosomes or homologues – two nearly identical versions of a chromosome.

Diploid cells have two of each type of chromosome ( 23 pairs or 46 in humans).

Chromosome Structure During S phase,

each of the 2 homologues replicates, resulting in identical copies called sister chromatids.

Chromatids remain connected at a linkage site called the centromere.

Cell Division

Interphase – Cells grow, DNA replicates and winds up tightly (condensation).

Cell Division

Mitosis – nuclear cell division in somatic cells. Prophase Metaphase Anaphase Telophase

Continuous process where stages flow one into the next.

Cell Division

Prophase – Chromosomes condense enough to be seen with a light microscope.

Cell Division

Metaphase – Alignment of the chromosomes along center of cell.

Cell Division

Anaphase – Separation of the chromatids.

Sister chromatids split apart – one moving to each pole.

Cell Division

Telophase – re-formation of the nuclei.

Cell Division

Cytokinesis – division of the cytoplasm.

Two complete, diploid cells.

Cytokinesis

In animal cells, the cell pinches in two. This produces a

cleavage furrow which deepens until the cell splits.

• In plants, a cell plate forms dividing the cell in two.

• Cellulose is laid down to form new cell wall.

http://www.youtube.com/watch?v=VGV3fv-uZYI

Reproduction

When organisms reproduce sexually, a cell from each parent fuses together.

In order to avoid a doubling of chromosome number, there must be a way to reduce the number of chromosomes in the gametes (eggs & sperm).

Meiosis

Meiosis is the special type of cell division that produces eggs and sperm.

In meiosis, a diploid cell with two sets of homologous chromosomes will divide so that the daughter cells are haploid and have one set of chromosomes.

Fertilization

One haploid egg and one haploid sperm can then fuse in the process called fertilization forming a diploid zygote.

Meiosis

Chromosomes have replicated during interphase just as in mitosis.

Meiosis actually consists of two separate divisions. Meiosis I – serves to separate the two

versions of the chromosome (homologues).

Meiosis II – serves to separate the two replicas of each version (sister chromatids).

Meiosis

Because there is only one replication of DNA but two cell divisions, each of the four daughter cells is haploid – has only one set of chromosomes.

Meiosis I

Prophase I – Chromosomes become visible. The 2 versions of

each chromosome pair up and exchange segments. This is called crossing over.

Meiosis I

Metaphase I –Chromosomes line up in the middle. Which

chromosome faces which pole is random. This is called independent assortment.

Meiosis I

Anaphase I Homologues are pulled apart and move

toward opposite poles. Sister chromatids NOT separated yet. Each pole has half as many chromosomes

(one set rather than two) as the original cell.

Telophase I – the chromosomes gather at the two poles and wait for the onset of meiosis II.

Meiosis II

After a brief interphase in which NO DNA synthesis occurs, meiosis II begins.

Meiosis II is just like mitosis except that the sister chromatids are no longer identical due to crossing over.

Meiosis II

Prophase II – nuclear envelopes break down.

Metaphase II – chromosomes line up in the middle of the cell.

Meiosis II

Anaphase II – sister chromatids move to opposite poles.

Telophase II – The nuclear envelope reforms around the four sets of daughter chromosomes.

Meiosis II

The resulting 4 daughter cells are haploid.

No 2 cells are alike due to crossing over.

In animals, these cells develop directly into gametes (eggs & sperm).

Unique Features of Meiosis

Synapsis – occurs when homologous chromosomes line up along their entire length.

Crossing over - occurs when segments of the chromosomes are then exchanged.

Unique Features of Meiosis

Another unique feature is that the homologous chromosomes separate in meiosis I rather than sister chromatids. Sister chromatids separate in

meiosis II. Meiosis is a reduction

division.

http://www.youtube.com/watch?v=D1_-mQS_FZ0&NR=1