bio 204 ch 11

Describe what an oncogene, a proto-oncogene, and a tumor suppressor gene do.

An oncogene is a gene that causes cancer. A proto-oncogene is a gene that has the potential to cause cancer when mutated. A tumor suppressor gene blocks specific steps in the development of cancer.

Name two ways in which meiotic cell division creates genetic diversity, and explain how each occurs.

Genetic diversity is created in meiotic cell division by the crossing over of chromosomes and random alignment of bivalents in metaphase I. In crossing over, homologous chromosomes of maternal origin and paternal origin exchange DNA segments, thus creating genetic diversity. Random alignment of bivalents leads to a chromosome set that is a random mix of maternal and paternal homologs.

Explain how cytokinesis differs between animal and plant cells.

In animal cells, cytokinesis involves a contractile ring made of actin. This contractile ring forms against the inner face of the cell membrane at the equator of the cell. When the ring contracts, the cytoplasm is pinched in half, forming two new daughter cells. In plant cells, cytokinesis involves the growth of a new cell wall called a cell plate in the middle of the dividing cell. The new plate fuses with the original cell wall at the perimeter of the cell and two new daughter cells are formed.

Describe the roles of cyclins and cyclin-dependent kinases in the cell cycle.

In the cell cycle, cyclins bind to and activate cyclin-dependent kinases (CDKs). Once activated, the CDKs then phosphorylate target proteins involved in promoting cell division.

Describe how chromosomes behave in meiosis. State when chromosomes are duplicated (forming sister chromatids) and when they are not duplicated.

Meiosis basically consists of two consecutive cell division cycles called meiosis I and meiosis II. Prior to meiosis, chromosomes are duplicated, forming sister chromatids. In meiosis I, homologous chromosomes pair, cross over, and separate into two different daughter cells. During meiosis II, sister chromatids separate into two more daughter cells. The result of this process is four haploid daughter cells, each having unique genetic information.

Compare and contrast mitotic cell division and meiotic cell division in terms of number of products, number of cell divisions, and the processes unique to each.

Mitosis is preceded by one round of DNA synthesis and produces two genetically identical daughter cells, each with 46 chromosomes, in one round of division. Meiosis is also preceded by one round of DNA synthesis, but consists of two rounds of cell division to produce four genetically different daughter cells, with 23 chromosomes each. See Fig. 11.12.

Compare and contrast the ways in which prokaryotic cells and eukaryotic cells divide.

Prokaryotic cells reproduce through a process called binary fission. During this process, the cell replicates its DNA, increases in size, and divides into two daughter cells, each having one copy of the parental DNA. Eukaryotic cells go through a similar process, albeit more complex, called mitotic cell division. In this process, cells first replicate their chromosomes in the nucleus. The nuclear envelope then dissolves and each pair of chromosomes is divided by connecting to the mitotic spindle. Once the two full sets of chromosomes are separated, a nuclear envelope forms around each one. The cell then goes through a process called cytokinesis, where it is split into two new daughter cells. Binary fission is like mitosis in many ways although it differs in a few key aspects: (1) The size and shape of the genetic material; bacteria have a single circular chromosome, whereas eukaryotic cells generally have multiple linear chromosomes. (2) The DNA of prokaryotes is attached to the plasma membrane, allowing separation of DNA into the daughter cells by cell growth. Eukaryotic cells have to first dissolve the nuclear membrane and then reform it to separate DNA.

Name the five steps of mitosis, and draw the structure and position of the chromosomes at each step.

The five steps in mitosis are: (1) prophase (chromosomes condense, centrosomes radiate spindle microtubules and migrate to opposite poles), (2) prometaphase (microtubules of the mitotic spindle attach to chromosomes), (3) metaphase (chromosomes align in center of cell), (4) anaphase (sister chromatids separate and travel to opposite poles), and (5) telophase (nuclear envelope re-forms and chromosomes decondense). See Fig. 11.5 for an illustration of the changes in the chromosomes at each step.

Give three examples of checkpoints that the cell monitors before proceeding through the cell cycle.

Three examples of checkpoints during the cell cycle are (1) the spindle assembly checkpoint, which is active before anaphase and ensures that all chromosomes are attached to the spindles; (2) the DNA damage checkpoint, which is active in the G1 phase and checks for DNA damage; and (3) the DNA replication checkpoint, which is active in G2 and makes sure all the DNA is replicated.

Describe three situations in which mitotic cell division occurs.

Three situations in which mitotic cell division occurs are in the development of multicellular organisms, maintenance and repair of organs and tissues, and asexual reproduction of unicellular eukaryotes.