Cell Cycle

There are two types of proteins that regulate and drive the cell cycle

protein kinases and proteolytic enzymes.

Chromosome

A gene is made up of DNA, which codes for one or more polypeptides. A chromosome is made up of many genes. The DNA in the chromosome is wrapped around histone and non-histone proteins. Before DNA synthesis, there is only one double stranded helix of DNA in each chromosome.

What can control the cell cycle?

1) Certain chemicals are present in the cytoplasm. These will drive the cell cycle. 2) The Cell-Cycle Control System directs the sequential events of a cell cycle. This system is a set of molecules that coordinates key events in the cell cycle. This acts as a built in clock. 3) It is important that cells divide only upon reaching a certain size. Cells need to be large enough to ensure that the daughter cells will contain the necessary machinery to survive. 4) A number of environmental factors such as depletion of nutrients, changes in temperature and pH can stop a cell from growing and dividing. Cells require growth factors at various stages of the cell cycle. For example: Platelet-derived growth factor (PDGF) is important in wound healing. This factor increases cell division at the site of a wound. 5) Internal Message: When the kinetochores are all attached with the KSF this activates Anaphase promoting complex (APC). The APC breaks down the protein that holds the sister chromatids together. 6) The density of cells can control cell division. Cells need space to divide as crowding inhibits cell division. This is called DENSITY-DEPENDENT INHIBITION. Related to density-dependent inhibition (anchorage dependence) is a requirement for the adhesion of cells to a substructure. Cells will stop dividing if they lose anchorage. Cancer cells do not exhibit density-dependent inhibition.

Prophase

1) Chromosomes condense; histone I and RNA molecules play an important role in the supercoiling of the chromosomes. The chromosomes get more compact and become visible. What we draw as chromosomes are the chromosomes after they condense. 2) Nucleoli disappear. The nucleolus is made up of different parts of different chromosomes. When the chromosomes condense, the chromosomal parts are pulled out of the nucleolus, and the nucleolus disappears. 3) Each chromosome is made up of two sister chromatids and these two chromatids are held together by a kinetochore. The kinetochore is the actual site of the insertion of the spindle threads. The kinetochore is made up of proteins and is a permanent part of the chromosome.

The spindle apparatus is composed of three different types of spindle fibers

1) Kinetochore spindle fibers: These attach from the kinetochore of the chromosome to the centrosome. 2) Polar spindle fibers: These attach one centrosome to the other centrosome. 3) Aster spindle fibers: These attach the centrosome to the cell membrane.

Prometaphase

1) Nuclear envelope fragments 2) Microtubules of the spindle can now invade the nucleus and interact with the chromosomes. 3) Spindle fibers extend from the poles to the equator.

There are three types of nonkinetochore microtubules:

1) Some microtubules radiate from the centrosome towards the metaphase plate without attaching to chromosomes. Others are too short to reach the metaphase plate. 2) Still others extend across the plate and overlap with nonkinetochore microtubules from the opposite pole of the cell. 3) Aster fibers: these extend from the centrosome to the cell membrane. Along with the polar spindle fibers, these are thought to anchor the centrosome in one place.

During meiosis, three things happen to the homologues

1) The homologues pair up. 2) The homologues exchange genetic information. This is called crossing over. 3) The newly scrambled chromosomes separate and go into different daughter cells in such a way that each daughter cell contains only one of each pair of homologues. These cells are called gametes or sex cells.

Genome

A cell's total DNA allotment. A human's genome is contained in 46 chromosomes.

Chromatid

After DNA synthesis, there are two identical DNA helices connected by a structure called the centromere. Each DNA helix is called a chromatid. After DNA synthesis, the chromosome is made up of two identical chromatids connected by a centromere (region) (kinetochore: actual structure). The centromeres act as handles. These chromatids are called sister chromatids.

Anaphase

Anaphase begins when an enzyme breaks down the protein that holds the sister chromatids together and the spindle apparatus starts pulling the kinetochores to the opposite poles (kinetochore microtubules shorten as chromosomes approach the poles). The daughter kinetochores move apart dragging the chromosomes (each now a single strand) to the poles. Two cells begin to form. Microtubules pull a chromosome towards a pole by losing protein subunits at their centrosome and at the + end (attached to the kinetochore). How this works is unknown. The nonkinetochore microtubules are responsible for elongating the whole cell along the polar axis during anaphase.

Prophase II

As in mitotic prophase, there are two sister chromatids attached to a centromere. The chromosomes condense, the nucleus disappears, and the spindle apparatus forms.

Cytokinesis in Plant Cells

At the time of telophase, small membranous vesicles filled with polysaccharides; formed in the Golgi complex, form on the metaphase plate. The vesicles continue to form until they are more or less continuous and form a double membrane, which is called the cell plate. The cell plate becomes impregnated with pectin and forms a cell wall. The cell plate forms across the midline of the plant cell where the old metaphase plate was located.

G2 or Gap phase 2

Between synthesis and mitosis. The mitotic spindle proteins are synthesized. (Mitotic Spindle: a structure that is involved with the movement of chromosomes during mitosis.)

Abnormal Cell Division

Cancer Cells.

Gametes

Cells that you form specifically for reproduction: sperm and egg

Metaphase II

Centromeres move to the metaphase plate during metaphase II.

Cytokinesis in Animal Cells

Cytokinesis usually begins with a cleavage furrow at the metaphase plate by an indentation in the surface of the cell. It looks as though the cell membrane were being pulled toward the middle, as if a thread were being wrapped around the cell and being tightened.

S Phase or the Synthesis Phase

DNA and chromosomal proteins are replicated. This phase lasts a few hours.

Regulation of the cell cycle

Different types of cells take different amounts of time to complete the cell cycle.

Meiosis II

Division of the chromosomes, analogous to mitosis.

Cytokinesis

Division of the cytoplasm.

Anaphase II

During anaphase II, the kinetochores are broken down by enzymes, and sister chromatids separate and move to the opposite poles

Telophase II

During telophase II, the nuclear membrane reforms and chromosomes decondense

cell cycle

G1 phase, S phase, G2 phase, and M phase.

Premeiotic Interphase

G1, S (replication of the chromosomes), and G2

Tumor cells

There is usually an abnormal chromosome number in tumor cells. The metabolism may be different and the cells cease to be constructive. The surface of the cells change, and they lose their ability to form attachments to neighboring cells and extracellular substructure. This process allows cancer to spread.

HeLa cells

Henrieta Lachs, an African American female who died in 1951. Her cancer cells still survive and are now world-wide. In fact, all of the cells together are 400 times Henrietta's body weight.

METASTASIS

If cancer cells enter the circulatory system (blood and lymph), then the cancer can spread to all parts of the body

ASTER

If the cells contain centrioles, a pair of newly formed centrioles marks each pole. The cells with centrioles have a third set of spindle fibers

Homologous Chromosomes:

In humans there are 46 chromosomes. Each chromosome consists of a double helix molecule of DNA. The DNA is folded with proteins to make up a chromosome. One chromosome represents hundreds or thousands of genes, and each gene is a specific region of the DNA molecule. A gene's specific location on the chromosome is called the its LOCUS. The 46 chromosomes are actually 23 pairs of chromosomes. The members of each pair are called homologous chromosomes (homologues). The two homologues are functionally equivalent and contain the same kinds of genes arranged in the same order.

Metaphase I

In the first metaphase, the tetrads are brought to the metaphase plate. The synaptonemal complex is lined up on the metaphase plate.

Meiosis in Males

In the male each of these haploid cells is called a spermatid. This spermatid will undergo cellular differentiation to become gametes (sperm)

Cytokinesis I

In this phase, the cytoplasmic division occurs

Telophase I

In this phase, the nucleus reorganizes and the nuclear membrane reforms. The chromosomes decondense.

crossing over possible problems

Insertion and deletion (this occurs with unequal crossing over. One chromosome will get too many DNA bases, and the other will not get enough). Inversion is also a problem. After crossing over, the homologs begin to pull away from each other, except at the crossing over points called the CHIASMATA (CHIASMA- singular)

Meiosis in Females

Meiosis is begun but is only partly completed in human females shortly before birth. All oocytes (oogonia) begin meiosis I and become primary oocytes. They remain frozen (in stasis) in the last stage of meiotic prophase I

Single cell organisms:

Mitosis allow for an increase in the population. This is a form of asexual reproduction. There is no exchange of genes between individuals. The colony will be made up of individuals with genes that are identical to the founder. These are called clones. This is called Binary Fission.

M phase

Mitosis and Cytokinesis. Mitosis is the division of the nucleus, which is usually followed by cytokinesis (division of the cytoplasm)

M stage (mitosis) has five phases

Prophase, Prometaphase, Metaphase, Anaphase, and Telophase

Somatic Cells

Regular body cells

Meiosis I

Replication of chromosomes, crossing over of the chromosomes and reduction in the chromosome number from diploid to haploid. Meiosis I is often called the reduction division.

Telophase

Reverse of prophase, but there are now two nuclei instead of one 1) Chromosomes decondense 2) Nuclear membrane reappears. 3) Spindle fibers become disorganized. 4) The cell pinches in the middle, beginning the formation of two cells

Proteolytic Enzymes

These enzymes break down proteins. Proteolytic enzymes controls the onset of anaphase by breaking down the protein that holds the sister chromatids together. They also breakdown cyclin thus deactivating the kinases.

Interphase

The Gap 1, Synthesis, and Gap 2 stages occurs first and prepares the cell for mitosis. During interphase, the cell grows, replicates the DNA and chromosomal proteins, and grows.

The Condensed Chromosomes

The chromosomes begin to condense after the S phase of the cell cycle. These chromosomes can be seen under the light microscope. Each chromosome consists of two copies of the same chromatid. Each chromatid is joined together by a constricted area common to both chromatids. This region of attachment is known as the centromere. Within the constricted region is a disc shaped protein containing structure-- the kinetochore. This is where the microtubules of the spindle are attached.

G1 Phase or the Gap 1 Phase

The chromosomes decondense as they enter the G1 phase; this is a physiologically active time for the cell. The cell synthesizes the necessary enzymes and proteins needed for cell growth. DNA consists of a single unreplicated helix (with histone and non-histone proteins). In the G1, the cell may be growing, active, and performing many intense biochemical activities.

Metaphase

The chromosomes line up on a plane called the metaphase plate. This lies in the middle of the spindle apparatus and is perpendicular to its axis. In actuality, the centromere/kinetochore is the only thing that lines up on the plate, the chromatids on the chromosomes can be pointing in any direction. At metaphase, the chromosomes are aligned on the cell's midline. Approximately 15-35 microtubules are attached to the kinetochore (by kinetochore microtubules

tetrads

The meiotic cell of a human now has 23 genetic entities each packet containing four chromatids and two centromeres. This is the point when crossing over occurs. A special enzyme causes the chromatids to unwind, revealing the strands of DNA. A complex series of events happen and the genetic material is exchanged between homologues. This is called the Holliday Junction.

Anaphase I

There is no separation of the centromeres, but the synaptonemal complex separates. This means that the homologues separate and move to opposite poles. The first meiotic division reduces the chromosome number by half.

Protein Kinases

These proteins activate and inactivate molecules by phosphorylating them. Specific protein kinases move molecules through the G1 and G2 checkpoints. Most of the time the kinases are inactive. They are activated when the kinases attach to the protein cyclin. They are called cyclin dependent kinases (cdk). There is an MPF complex (Maturation Promoting Factor or M phase Promoting Factor) that moves the cell from late G2 (interphase) to mitosis. The level of MPF increases and decreases-- MPF appears in late interphase and reaches the highest concentration during mitosis. MPF disappears after mitosis. MPF acts as a protein kinase and leads to the phosphorylation of certain chromatin proteins that cause the chromosomes to condense during prophase. MPF also stimulates the proteins in the nuclear lamina and other kinases, which fragments the nuclear envelope.

Meiotic Interphase

This involves G1 and G2 phases only. There is no S phase in this interphase. This phase may be brief or last a long time.

Meiotic Prophase I

This is long and complex compared with mitotic prophase. In it: 1) Nuclear membrane disappears. 2) Spindle fibers form. 3) The chromosomes condense. 4) The homologous chromosomes pair up by touching each other in the appropriate places. First there is a lot of random movement of chromosomes until the homologous chromosomes find each other. It is important, for example, that chromosome #13 finds homologous chromosome #13. When the two homologous touch each other in the same place, a specialized structure called the Synaptonemal Complex holds the homologues together.

R (restriction) point

When normal cells stop growing due to changes in the environment or when touching other cells, they stop growing during the G1 phase of the cell cycle. Once a cell passes through the R point, the cell is committed to the M phase. If the cell doesn't divide, it enters the G0 phase or the nondividing state. The R point acts as a 'check point.'

meiosis

a process in which a diploid or double set of chromosomes is reduced to a haploid (n), or a single, set of chromosomes It is a process that guarantees that the number of chromosomes remains stable from generation to generation. In humans the diploid number = 46 (2n = 46), the haploid number = 23 (n = 23); in fruit flies: 2n = 8, n = 4. Multicellular organisms usually have two types of cells: 1) somatic and 2) gametes. Somatic cells are any type of cell that is not a sperm or egg. These cells are diploid (2n). Gametes are cells that are sperm or eggs. Gametes are haploid (n).

Cancer cells

do not respond to normal cell division controls. They divide excessively and ignore density-dependent inhibition.

Mitosis

duplication and division of the nucleus and the chromosomes contained there in, usually followed by cytokinesis.

Degree of freedom

number of categories - 1

Crossing over

occur at the INTRONS. Recombinases are enzymes that will recognize specific DNA sequences where recombination will take place. These enzymes will bring the specific sites together to form a protein-DNA complex—Holliday junction. Several thousand base pairs of one strand pairs with the chromatid on another homologues. There are breakages and the chromatids untangle themselves. Meanwhile other enzymes are repairing the breaks in the DNA. This process makes new chromatids and is a source of genetic variation within a population. The products are spliced together and is called a cross over product. The DSB (double-stranded break-repair pathway) plays an active role in recombination. Interestingly, without recombination, the chromosomes will not align properly for the first meiotic division. This will increase non-disjunction and increase infertility.

One set of chromosomes that does not occur as homologue

occurs in males; the X chromosome and the Y chromosome are not homologues but pair up in meiosis. In females, there are two X chromosomes that are homologues. These chromosomes are the sex chromosomes and the other 22 pairs of chromosomes are called AUTOSOMES.

diploid

organism has a complete set of chromosomes in every cell and is 2n (diploid means 'double set')

The X2 test

see if the data you collected falls into the acceptable range of the theoretical data (perfect data).

Cytokinesis II

the cytoplasm divides.

The spindle

the spindle apparatus which moves the chromosomes consists of two proteins: actin and tubulin. At the beginning of mitosis, the two centrosomes (centrioles in animal cells) that were fairly close together move to the opposite poles of the nucleus. As the nuclear membrane disappears, the spindle forms between the two centrosomes. During interphase, the centrosome duplicates to form two centrosomes just outside of the nucleus.

If cells divide too rapidly

they invade specialized tissues. This disrupts the function of the tissue and is referred to as cancer.

The first step in cancer cell formation

transformation, which is the conversion of a normal cell to a cancer cell. Usually the immune system destroys such cells. However, sometimes the cell can evade destruction. It will divide to form a tumor (a mass of cells in normal tissue). If the cells remain at the original site, it is a benign tumor. Malignant tumors spread.