The Cell Cycle and Cell Division

7

Chapter 7 The Cell Cycle and Cell Division

Key Concepts

• 7.2 Binary Fission and Mitosis

• 7.3 Cell Reproduction Control

Two types of Reproduction

Asexual Reproduction

• Faster

• Creates genetically identical cells (clones)

• Common in nature

• Only 1 parent is involved

• Examples: binary fission in prokaryotes and mitosis in eukaryotes

Sexual Reproduction

• Creates gametes (sex cells – sperm and egg cells)

• Product has genetic diversity

• 2 parents are involved

• Examples - meiosis

Asexual Reproduction on a Large Scale

Sexual Reproduction on a big scale

Basic Term - Chromosome

• Consist of DNA and protein

• 3 forms• Chromatin – 1 long strand, wrapped around

histones• Chromatid – half of a chromosome• Chromosome – full X, 2 sister chromatids held

together by a centromere• Contain specialized proteins called

kinetochores

• Homologous Chromosomes – in pairs, similar in shape, size and information (1 from Mom and 1 from Dad) - TETRAD

Basic Term - Chromosome

Figure 7.5 The Phases of the Eukaryotic Cell Cycle (Part 1)

Two kinds of Cells

Diploid Cells

• Chromosomes are in pairs

• Abbreviated 2n

• Example – Somatic (body) cells, zygote

• Humans diploid # = 46

Haploid Cells

• Chromosomes are not in pairs

• Abbreviated 1n

• Example – Gametes/Sex Cells/Sperm/Egg/Spores

• Human haploid # = 23

Karyotype of Homo sapiens

Mitotic (doubled) chromosomes taken from a white blood cell at metaphase

23 chromosome pairs, 46 total

Stained with Giemsa stain to reveal differences in the DNA/protein associations.

Banding distinctive to each chromosome

What was the sex of this individual?

For any cell to divide

Four events must occur for cell division:

• Reproductive signal—to initiate cell division

• Replication of DNA

• Segregation—distribution of the DNA into the two new cells

• Cytokinesis—division of the cytoplasm and separation of the two new cells

Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells

In prokaryotes, cell division results in reproduction of the entire organism.

The cell:

• Grows in size

• Replicates its DNA

• Separates the DNA and cytoplasm into two cells through binary fission

Cell Cycle

• 5 parts

1.G1 - growth

2.S – DNA Synthesis

3.G2 – prepare for mitosis

4.M – mitosis (PMAT)

5.C – Cytokinesis – division of the cytoplasm

Interphase

Interphase

• 3 parts – G1,S,G2

• Nucleolus and Nuclear Membrane is Visible

• Longest phase of the cell cycle

• Chromatin is visible

Figure 7.5 The Phases of the Eukaryotic Cell Cycle (Part 2)

Prophase

• 1st stage of mitosis

• Chromatin chromosomes

• Centrosome and spindle fibers are visible and start to organize and move to opposite poles

• Pair of centrioles

• Nuclear Membrane and Nucleolus disappear

Metaphase

• 2nd stage of mitosis

• Chromosomes are lined up at the equator of the cell, connected to the spindle fibers by their centromere

Anaphase

• 3rd phase in mitosis

• Chromosomes chromatids

• Spindle fibers pull the chromosomes apart and move them towards opposite poles

Telophase

• Last phase of mitosis

• Chromatids chromatin (less compact)

• Nuclear membrane and nucleolus return

• Overlaps with Cytokinesis

Cytokinesis

• After Interphase

• Division of the cytoplasm (cleavage furrow vs. cell plate)

• Each daughter cell contains an exact copy of DNA and are identical in every way

Figure 7.6 The Phases of Mitosis (1)

Figure 7.6 The Phases of Mitosis (2)

Concept 7.3 Cell Reproduction Is Under Precise Control

• The reproductive rates of most prokaryotes respond to environmental conditions.

• In eukaryotes, cell division is related to the needs of the entire organism.

• Cells divide in response to extracellular signals, like growth factors.

Concept 7.3 Cell Reproduction Is Under Precise Control

• Progression is tightly regulated—the G1-S transition is called R, the restriction point.

• Passing this point usually means the cell will proceed with the cell cycle and divide.

Concept 7.3 Cell Reproduction Is Under Precise Control

• Specific signals trigger the transition from one phase to another.

• Transitions also depend on activation of cyclin-dependent kinases (Cdk’s).

• A protein kinase is an enzyme that catalyzes phosphorylation from ATP to a protein.

• Phosphorylation changes the shape and function of a protein by changing its charges.

Concept 7.3 Cell Reproduction Is Under Precise Control

Cdk is activated by binding to cyclin (by allosteric regulation); this alters its shape and exposes its active site.

The G1-S cyclin-Cdk complex acts as a protein kinase and triggers transition from G1 to S.

Other cyclin-Cdk’s act at different stages of the cell cycle, called cell cycle checkpoints.

Figure 7.10 Cyclins Are Transient in the Cell Cycle

Concept 7.3 Cell Reproduction Is Under Precise Control

Example of G1-S cyclin-Cdk regulation:

Progress past the restriction point in G1 depends on retinoblastoma protein (RB).

RB normally inhibits the cell cycle, but when phosphorylated by G1-S cyclin-Cdk, RB becomes inactive and no longer blocks the cell cycle.

RB + ATP RB-P + ADPActive – blocks cell cycle Inactive – allows cell cycle to go forward