Homework questions to be emailed to valenciabiologyhw@gmail.com

  1. Describe how chromosome number changes throughout the human life cycle
  2. Compare cytokinesis in animals and plants
  3. Describe the roles of checkpoints, cyclin, Cdk, and MPF in the cell-cycle control system
  4. Describe the process of synapsis during prophase I and explain how genetic recombination occurs
  5. List sources of genetic variation

 

Mitosis and Meiosis.

Ch 12 & 13    

Chromosome--The term was proposed for the individual threads within a cell nucleus (gk. chroma, colour; soma, body). The self-replicating genetic structures of cells containing the cellular DNA that bears in its nucleotide sequence the linear array of genes. In prokaryotes, chromosomal DNA is circular, and the entire genome is carried on one chromosome. Eukaryotic genomes consist of a number of chromosomes whose DNA is associated with different kinds of proteins.

Genome-All the genetic material in the chromosomes of a particular organism; its size is generally given as its total number of base pairs

Mitosis-The most frequent process of nuclear division (karyokinesis) in cells that produces daughter cells that are genetically identical to each other and to the parent cell. The mitosis is divided into four (or five) phases: prophase, prometaphase, metaphase, anaphase, telophase. Mitosis and interphase make the cell cylcle.

Diploid A full set of genetic material, consisting of paired chromosomes one chromosome from each parental set. Most animal cells except the gamete have a diploid set of chromosomes. The diploid human genome has 46 chromosomes

Polyploid  Having three or more (Gk. polys, many) sets of homologous chromosomes

Bacteria DNA-1 chromosome splits, attaches to membrane and pulls apart.

 

What is MitosisTHE PROCESS BY WHICH TWO NEW NUCLEII ARE FORMED, PRECEDING CELL DIVISION

1 chromosome (duplicated) splits into two identical chromosomes

DNA Replicating and thus the chromosome replicating from 1 strand into 2 identical strands…A goes wit T and C goes with G

INTERPHASE is the

normal lifetime of a

cell, after being

born” by division,

and before it divides

itself.

Prokaryotes-chromosomes move apart to opposite ends of cell..maybe attached to cell membrane.  Dinoflagellates-nuclear envelope remains intact, chromosomes attach to envelope, microtubules pass through nucleus and pull it apart.

Diatoms the envelope remains intact but microtubules form a spindle inside nucleus which separate the chromosomes and the nucleus splits into 2.

Eukaryotes, spindle forms outside of nucleus and envelope breaks down during mitosis, reforming afterwards.

Meiosis is a specialized type of cell division that occurs in the formation of gametes such as egg and sperm. Although meiosis appears much more complicated than mitosis, it is really just two divisions in sequence, each one of which has strong similarities to mitosis.

MEIOSIS IS THE PROCESS

OF CUTTING BY HALF THE

NUMBER OF CHROMOSOMES

FOUND IN GAMETES.

homologous chromosomes 

Meaning: chromosomes, that are similar in size and genetic constitution; one member of each pair of homologous chromosomes comes from the mother and the other from the father.

Every cell

has pairs

of chromo-

somes

 

 

(except

gametes).

1. What is meiosis ?

2. Explain why meiosis is necessary.

3. Name the only 2 kinds of human cells that do NOT have a full 2N set.

4. What is meiosis in females called ? males ?

5. Where does meiosis in females occur ?

6. Where does meiosis in males occur ?

7. Give the mathematical formula for fertilization using chromosome numbers.8. What would to chromosome numbers after a few generations without meiosis ?

 

 

 

The Cell Cycle Control

The timing and rate of cell division differ between different organisms and also between different cells of an organism.

Compare skin cells with muscle or nerve cells.

What is controlling the rate of cell division, how cells "know" that it is time to divide? Why cancer cells do not stop dividing?

 

The cell cycle is regulated at the molecular level

Accumulating experimental evidences appearing since early seventies suggested that the cell cycle is driven by specific chemical signals present in the cytoplasm.

Most of the experiments were conducted with cell cultures.

Many types of animal and plant cells can be removed from an organism and cultured in an artificial environment.

 

 

Cell cycle is controlled

Cultured mammalian cells can be induced to fuse, forming a single cell with two nuclei.

The results of fusing cells at two different phases of the cell cycle suggested that particular chemicals control the progression of phases.

 

For example, when a cell in M phase was fused with one in any other phase, the nucleus from the latter cell immediately began mitosis.

If the second cell was in Gl, the condensed chromosomes that appeared had single chromatids.

 

 

Short summary of interphase

·     Comprised by three (four) subphases: G1, (GO), S and G2.

           Common feature of all (except GO) subphases: growth of the cell by producing proteins and _ cytoplasmic organelles.

·         G 1- "first gap"

·         S phase - chromosome duplication

G2 - "second gap", DNA repair

 

Cell cycle control

A checkpoint in the cell is a critical control point where stop and go signals can regulate the cycle

Animal cells have built-in "stop" signals that halt the cell cycle at checkpoints until overridden by "go" signals

 The most important decision to make is: to continue the cell division after the exit from M phase or not.

 

Cells that do not receive the "go" signal at the G 1 checkpoint, switch into a nondividing state called the GO phase.

A good example of these cells are liver cells.

They can be called back to the cell cycle by growth factors released during injury.

 

 

 

Cyclins and cyclin-dependent kinases (Cdks)

Regulatory molecules of the cycle transition are proteins of two main types: protein kinases and cyclins.

 Protein kinases are proteins that regulate the activity of the others by phosphorylating them. "Go" signal at the G 1 and G2 checkpoints is regulated by particular protein kinases.

 

 

To be active, such a kinase must be attached to a cyclin, a protein that gets its name from its cyclically fluctuating concentration in the cell .

This kinases are called cyclin-dependent kinases - Cdks

Cdk-cyclin complex called MPF, acts at the G2 checkpoint to trigger mitosis.

 

 

 

 

 

            The graph shows how MPF activity fluctuates with the level of cyclin in the cell. The cyclin level rises throughout interphase (G 1, S, and G2 phases), then falls abruptly during mitosis (M phase). The Cdk itself is present at a constant level.

(b)1. By the G2 checkpoint (red bar), enough cyclin is available to produce many molecules of MPF.

 

 

2. MPF promotes mitosis by phosphorylating various proteins, including other enzymes.

3. One effect of MPF is the initiation of a sequence of events leading to the breakdown of its own

cyclin.

4. The Cdk component of MPF is recycled