9Chromosomes, the Cell Cycle,

and Cell Division

9 Systems of Cell Reproduction

• The continuity of life is based on the reproduction of cells.

• All the cells in a multicellular organism originate from a single cell (zygote).

• Cell division is necessary for:

Reproduction

Growth and

Repair of an organism

9 Cell Division

In order for cell division to occur:

• A signal to reproduce must be received.

• Replication of DNA and other cell components must occur.

• Distribution of replicated DNA

• Cytokinesis

9 Prokaryotes

Have one circular chromosome.

Prokaryotes grow in size, replicate DNA and divide into 2 cells in a process called cell fission.

Initiation of fission is controlled by environmental conditions.

Cytokinesis separates the one cell into two, each with a complete chromosome.

Figure 9.2 Prokaryotic Cell Division

9 Eukaryotes

• Eukaryotic cells divide by mitosis or meiosis.

• The reproduction of eukaryotic cells involves:

Replication of the DNA

Segregation of the replicated DNA into two new nuclei (nuclear division)

Division of the cytoplasm (cytokinesis)

• Eukaryotes usually have many chromosomes.

• Eukaryotes have a nucleus, which must replicate and, with few exceptions, divide during cell division.

9 Cell Cycle

• The cell cycle has two phases: mitosis and interphase.

• Interphase is the period between divisions.

• A typical eukaryotic cell will spend most of its life in interphase which consist of G1, S and G2. DNA is replicated during the S phase.

• Most of a cell’s life is spent in interphase.

• Human nerve and muscle cells, lose the capacity to divide altogether and stay in interphase indefinitely, while other cells divide regularly or occasionally.

Figure 9.3 The Eukaryotic Cell Cycle

9 Cell Cycle

• Transitions from G1 to S and G2 to M depend upon a protein called cyclin-dependent kinase, or Cdk.

• Cdk is activated by binding to a second type of protein called cyclin.

• Cyclin-Cdk complexes act as checkpoints. They allow or prevent the passage to the next cell cycle stage.

• In cancer cells, these cyclin-Cdk controls are often disrupted.

Figure 9.4 Cyclin-Dependent Kinases and Cyclins Trigger Transisions in the Cell Cycle

9 Eukaryotic Chromosomes

• Chromatin is a DNA protein complex.

• After DNA replication (S phase), the chromatin material condenses by wounding around histone cores.

• DNA folds repeatedly becoming densely coiled and folded.

• Each duplicated chromosome consists of two chromatids. They are joined at a single point called the centromere.

Figure 9.5 Chromosomes, Chromatids, and Chromatin

Figure 9.6 DNA Packs into a Mitotic Chromosome

9 Mitosis: Distributing Exact Copies of Genetic Information

• DNA and centrosomes duplicate in the S phase.

• In animal cells each centrosome contains two centrioles.

• During G2-to-M transition, the two centrosomes separate and move to opposite ends of the nuclear envelope.

• Centrosomes are regions where aster rays and spindle fibers are formed.

9 Mitosis

Mitosis is divided into phases for ease of study.

• Prophase• Preometaphase• Metaphase• Anaphase and • Telephase

9

Figure 9.8 – Part 1

Figure 9.8 – Part 1

figure 09-08a.jpg

9

Figure 9.8 – Part 2

Figure 9.8 – Part 2

figure 09-08b.jpg

9 Mitosis: Distributing Exact Copies of Genetic Information

• Prophase marks the beginning of mitosis.

Chromosomes condense and appear as chromatids.

The chromatids are held together by the centromere.

Nucleoli disappear.

Spindle fibers form between the two centrosomes.

Radiating from the centrosome are aster rays.

Kinetochores develop.

Figure 9.8 Mitosis (Part 1)

9• During prometaphase the nuclear envelope

disappears – it breaks into small vesicles, permitting the fibers of the spindle to “invade” the nuclear region.

• The chromosomes move toward the middle of the spindle.

• The spindle fibers attach to the chromatids at the kinetochores.

Prometaphase

9 Metaphase

During metaphase,

• the double chromsomes (consisting of 2 chromatids) aggregate at the equatorial plate.

• the double chromosomes are readily distinguishable. Karyotypes are done using chromosomes from this phase.

• the kinetochores attach to the spindle fibers.

• at the end of metaphase, all chromatid pairs separate simultaneously.

Figure 9.8 Mitosis (Part 2)

9

• Anaphase begins when the chromatids separate.

• Motor proteins and shortening of kinetochore move a chromatid (now referred to as a chromosome) to opposite poles.

• When chromosomes top moving the cell enters telophase.

Anaphase

9• The chromosomes uncoil to form tangled

chromatin.

• Nuclear envelopes and nucleoli re-form forming two nuclei whose chromosomes are identical to each other and to those of the cell that began the cycle.

Telophase

9 Cytokinesis: The Division of the Cytoplasm

• Animal cells divide by a furrowing (a “pinching in” or constriction) of the plasma membrane.

• In plant cells, cytokinesis is accomplished by vesicle fusion (from the Golgi apparatus) producing a cell plate.

Figure 9.10 Cytokinesis Differs in Animal and Plant Cells

9 Reproduction: Asexual and Sexual

• Mitosis can repeat many times resulting in vast numbers of genetically identical cells.

• Mitosis video

9 Reproduction: Asexual and Sexual

• It is possible to stain and photograph chromosomes during metaphase.

• A photograph of the slide can be taken, and images of each chromosome can be organized based on size, shape, banding patterns, and location of centomere.

• This organization is called a karyotype.

9Figure 9.13 Human Cells Have 46 Chromosomes

9 Control of Cell Division

• Proteins form Cdks complexes that control all stages of the cell cycle.

• Cdk complexes are composed of cyclin, a protein whose concentration cyclically fuctuates, and a kinase.

9 Control of Cell Cycle

Cyclin-Cdk complexes act as checkpoints• If DNA is damaged by UV radiation, p21

stops the cell cycle until DNA is repaired.• Cyclin-Cdk defects have been found in

cancer cells. A breast cancer with too much cyclin D

has been found. Protein 53, which inhibits activation of

Cdk is found defective in ½ of all human cancers.

9 Control of Cell Division

• External controls also influence the cell cycle.

• Growth factors – a growth factor is a protein that stimulates other cells to divid.

• Density-dependent inhibition.• Anchorage dependence.

9 Reproduction: Asexual and Sexual

• Asexual reproduction produces a new organism genetically identical to the parent.

• Any genetic variety is due to mutations.

9 Sexual Reproduction

• Two gametes each containing ½ of the number of chromsomes found in other cells combine

• A single cell is formed called the zygote, or fertilized egg.

• Fosters genetic diversity.

• Meiosis is involved.

9 Genetic Diversity

• In each pair of chromosomes one chromosome comes from the mother and the other from the father.

• The members of the pair are called homologous chromosomes The chromosomes are similar in size and appearance but different in DNA composition.

• Haploid cells, 1n, contain 1 homolog from each pair. Example – gametes

• When haploid gametes fuse in fertilization, they create the zygote, which is 2n, or diploid.

9 Meiosis: A Pair of Nuclear Divisions

• Meiosis consists of two nuclear divisions that reduce the number of chromosomes to the haploid number. The DNA is replicated only once.

• The functions of meiosis are:

To reduce the chromosome number from diploid to haploid.

To ensure each gamete gets a complete set of chromosomes.

To promote genetic diversity among products.

9

Figure 9.14 – Part 1

Figure 9.14 – Part 1

figure 09-14a.jpg

9

Figure 9.14 – Part 2

Figure 9.14 – Part 2

figure 09-14b.jpg

9 Meiosis: A Pair of Nuclear Divisions

• Meiosis I is preceded by an interphase in which DNA is replicated.

• During prophase I, synapsis occurs: The two homologs are joined together by a complex of proteins.

• Genetic material may be exchanged between homologous chromosomes - crossing-over.

• This crossing-over increases genetic variation by reshuffling the genes on the homologs.

Figure 9.14 Meiosis (Part 1)

Figure 9.16 Crossing Over Forms Genetically Diverse Chromosomes

9 Meiosis: A Pair of Nuclear Divisions

• In metaphase I, the paired homologs gather at the equator.

• The homologous chromosomes separate in anaphase I.

• The individual chromosomes are pulled to the poles, with one homolog of a pair going to one pole and the other homolog going to the opposite pole.

• Result is two nuclei each with the haploid number of chromosomes with two sister chromatids.

Figure 9.14 Meiosis (Part 2)

Figure 9.14 Meiosis (Part 3)

9 Meiosis: A Pair of Nuclear Divisions

• The second meiotic division separates the chromatids.

• Meiosis II is similar to mitosis however DNA does not replicate before meiosis II.

• In meiosis II, there is no crossing-over.

• Result is 4 cells each with a haploid number of chromosomes.

Figure 9.14 Meiosis (Part 4)

Figure 9.14 Meiosis (Part 5)

Figure 9.14 Meiosis (Part 6)

9 Meiosis: A Pair of Nuclear Divisions

• Meiosis leads to genetic diversity.

• Synapsis and crossing-over during prophase I mix the genetic material.

• Which member of a homologous pair segregates or goes to which daughter cell at anaphase I is a matter of chance. This phenomenon is called independent assortment.

9 Nondisjunction

• Nondisjunction can occur when homologous chromosomes fail to separate during anaphase I, or.

• if sister chromatids fail to separate during anaphase II

Figure 9.18 Nondisjunction Leads to Aneuploidy

9 Meiotic Errors

• Failure of chromosome 21 to separate in humans results in trisomy 21—Down syndrome.

• Translocation, a process in which part of a chromosome attaches to another, can also cause abnormality.

9 Meiotic Errors

• Polyploids have extra whole sets of chromosomes.

• Triploids are 3n; tetraploids are 4n.

• Mitosis is usually nonproblematic, meiosis is problematic, especially for odd numbers of chromosomes, as in triploidy.

9 Cell Death

• Cells die when they are damaged by poisons or are starved of essential nutrients. These cells swell and burst or

• Cells may self-destruct

The cell may no longer be needed, e.g., cells of the web-like between the fingers of a fetus.

Old or damaged cells may need to be replaced.

The cell death cycle is controlled by signals.

Cells may detach from their neighbors fragments its own DNA, and gets ingested by surrounding living cells.