Chapter 10: How Cells Divide

Interphase: Preparation for Mitosis

After the S phase, the sister chromatids appear to share a common centromere, but at the molecular level the DNA of the centromere has actually already replicated, so there are two complete DNA molecules. The two chromatids are held together by cohesion proteins at the centromere, and each chromatid has its own set of kinetochore proteins (an attachment site for microtubules necessary to separate the chromosomes during cell division) After the chromosomes have replicated in S phase, they remain fully extended and uncoiled, although cohesin proteins are associated with them at this stage. In G2 phase, they begin the process of CONDENSATION, coiling even more tightly. Also during G2 phase, the cells begin to assemble the machinery they will later use to move the chromosomes to opposite poles of the cell. CENTRIOLES, a pair of microtubule- organizing centers, replicate producing one for each pole.

Bacterial Cell Division

Bacterial cells divide by BINARY FISSION. - prior to cell division, the single, circular, bacterial DNA molecule replicates. - replication begins at the origin of replication. - the replicated enzymes move out in both directions from the origin of replication. - new chromosomes are partitioned to opposite ends of the cell. - a SEPTUM forms to divide the cell into 2 cells.

Chromosome Replication

Chromosome must be replicated before cell division. After replication, each chromosome is composed of two identical DNA molecules held together by a complex of proteins called COHESINS (complex of proteins holding replicated chromosomes together). As the chromosomes become more condensed and arranged about the protein scaffold, they become visible as two strands that are held together at the CENTROMERE. At this point, we still call this one chromosome, but it is composed of two SISTER CHROMATIDS (2 copies of the chromosome within the replicated chromosome).

Chromosome Structure

Chromosomes are very long and must be condensed to fit within the nucleus. Every 200 nucleotides, the DNA duplex is coiled around a core of 8 HISTONE PROTEINS- positively charged proteins due to an abundance of the amino acids arginine and lysine. The complex of DNA and histone proteins are called the NUCLEOSOME. The DNA wrapped in nucleosomes is further coiled into an even more compact structure called the SOLENOID- the 30 nm fiber. This is the usual state of nondividing chromatin. The solenoid is further compacted: - radical loops are held in place by scaffold proteins - scaffold of proteins is aided by a complex of proteins called condensin - occurs during mitosis

Prometaphase

Chromosomes become attached to the spindle apparatus by kinetochores. A second set of microtubules is formed from the poles to each kinetochore. Microtubules begin to pull each chromosome towards the center of the cell.

Prophase

Chromosomes continue to condense. Centrioles move to each pole of the cell. Spindle Apparatus is assembled. Nuclear envelope dissolves. Nucleolus (where ribosomes are produced) disappears.

Eukaryotic Cell Cycle

Five phases of the cycle: 1. G1 (gap phase 1) the primary growth phase of the cell. The term gap phase refers to its filling the gap between cytokinesis and DNA synthesis. For most cells, this is the longest phase. 2. S (synthesis) is the phase in which the cell synthesizes a replica of the genome. (DNA replication) 3. G2 (gap phase 2) is the second growth phase, and preparation for separation of the newly replicated genome. This phase fills the gap between DNA synthesis and the beginning of mitosis. During this phase microtubules begin to reorganize to form a spindle. Chromosomes condense. * G1, S, and G2 together constitute INTERPHASE, the portion of the cell cycle between divisions. 4. MITOSIS is the phase of the cell cycle in which the spindle apparatus assembles, binds to the chromosomes, and moves the sister chromatids apart. Mitosis is the essential step in separation of the two daughter genomes. It is traditionally subdivided into five stages: prophase, prometaphase, metaphase, anaphase, and telophase. 5. CYTOKINESIS is the phase of the cell cycle when the cytoplasm divides, creating two daughter cells. In animal cells, the microtubule spindle helps position a contracting ring of actin that constricts like a drawstring to pinch the cell in two. In cells with a cell wall, such as plant cells, a plate forms between the dividing cells. * Mitosis and cytokinesis together are usually referred to collectively as M phase, to distinguish the dividing phase from interphase. * The duration of the cell cycle varies depending on cell type.

Eukaryotic Chromosomes

Linear chromosomes. Chromosome number vary among different species- human have 46 chromosomes, consisting of 23 nearly identical pairs. Chromosomes are composed of CHROMATIN- complex of DNA and protein. - HETEROCHROMTIN: not expressed - EUCHROMATIN: expressed

Metaphase

Microtubules pull the chromosomes to align them at the center of the cell. METAPHASE PLATE- imaginary plate through the center of the cell where the chromosomes align.

Anaphase

Removal of cohesin proteins causes the centromeres to separate. Microtubules pull sister chromatids towards the poles. In ANAPHASE A the kinetochore are pulled apart. In ANAPHASE B the poles move apart.

Telophase

Spindle apparatus disassembles. Nuclear envelope forms around each set of sister chromatids. Chromosomes begin to uncoil. Nucleolus reappears in each new nucleus.

Cytokinesis

The cleavage of the cell into equal halves. Animal Cells: construction of actin filaments produces a CLEAVAGE FURROW. Plant Cells: plasma membrane forms a CELL PLATE between the nuclei. In fungi and some protists: mitosis occurs within the nucleus; division of the nucleus occurs with cytokinesis.

Chromosome Karyotypes

The particular array of chromosomes an individual organism possesses.

Cancer

The unrestrained, uncontrolled growth of cells in humans leads to the disease, cancer. Cancer is essentially a disease of cell division- a failure of cell division control. Two kinds of genes can disturb the cell cycle when they are mutated: 1. Tumor-suppressor gene: - prevent the development of many cells containing mutations. - for example, p53 halts cell division if damaged DNA is detected. - p53 is absent or damaged in many cancerous cells. 2. Proto-oncogenes: - some encode receptors with growth factors. - some encode signal transduction proteins. - become ONCOGENES when mutated. - oncogenes can cause cancer when they are introduced into a cell.

Control of the Cell Cycle

There are two irreversible points in the cell cycle: 1. the replication of genetic material 2. the separation of the sister chromatids. The cell uses three checkpoints to both assess its internal state and integrate external signals: 1. G1/S Checkpoint: the cell "decides" to divide. 2. G2/M Checkpoint: the cell makes a commitment to mitosis. 3. Late metaphase (spindle) Checkpoint: the cell ensures that all chromosomes are attached to the spindle.

The kinetochhore is a structure that functions to a. connect the centromere to microtubules. b. connect centrioles to microtubules. c. aid in chromosome condensation. d. aid in chromosome cohesion.

a

Chromatin is composed of a. RNA and proteins. b. DNA and proteins. c. sister chromatids. d. chromosomes.

b

Genetically, proto-oncogenes act in a dominant fashion. This is because a. there is only one copy of each proto-oncogenes in the genome. b. they act in a gain-of-function fashion to turn on the cell cycle. c. they act in a loss-of-function fashion to turn off the cell cycle. d. they require that both genomic copies altered to affect function.

b

What is the role of cohesin proteins in cell division? a. They organize the DNA of the chromosomes into highly condensed structures. b. They hold the DNA of the sister chromatids together. c. They help the cell divide into two daughter cells. d. They connect microtubules and chromosomes.

b

Why is cytokinesis an important part of cell division? a. It is responsible for the proper separation of genetic information. b. It is responsible for the proper separation of the cytoplasmic contents. c. It triggers the movement of a cell through the cell cycle. d. It allows cells to halt at checkpoints.

b

Separation of the sister chromatids occurs during a. prophase. b. prometaphase. c. anaphase. d. telophase.

c

The bacterial SMC proteins, eukaryotic cohesin proteins, and condensin proteins share a similar structure. Functionally, they all a. interact with microtubules. b. can act as kinase enzymes. c. interact with DNA to compact or hold strands together. d. connect chromosomes to cytoskeletal elements.

c

The main difference between bacterial cell division and eukaryotic cell division is that a. since bacteria only have one chromosome, they can count the number of copies in the cell. b. eukaryotes mark their chromosomes to identify them and bacteria do not. c. bacterial DNA replication and chromosome segregation are concerted processes but in eukaryotes they are separated in time. d. None of the above is correct.

c

Binary fission in prokaryotes does not require the a. replication of DNA. b. elongation of the cell. c. separation of daughter cells by septum formation. d. assembly of the nuclear envelope.

d

The metaphase to anaphase transition involves a. new force being generated to pull the chromatids apart. b. an increase in force on sister chromatids to pull them apart. c. completing DNA replication of centromeres allowing chromosomes to be pulled apart. d. loss of cohesion between sister chromatids.

d

What steps in the cell cycle represent irreversible commitments? a. The S/G2 checkpoint b. The G1/S checkpoint c. Anaphase d. Both b and c are correct.

d

Mitosis

divided into 5 stages: 1. Prophase 2. Prometaphase 3. Metaphase 4. Anaphase 5. Telophase