3.a.2 – in eukaryotes, heritable information is passed to the next generation via processes that...

Chapter 12 The Cell Cycle

Essential Knowledge

3.a.2 – In eukaryotes, heritable information is passed to the next generation via processes that include the cell cycle and mitosis, or meiosis plus fertilization (12.1-12.3).

Roles of Cell Division

1. Reproduction (uni vs. multi)2. Growth3. Repair/replace old or damaged cells4. To distribute identical genetic material

to two new daughter cells

Goal: To split the sister chromatids and give one to each new cell – to make two new cells with the correct amount of genetic info

Two cell types

1. Somatic Non-sex cells (all body cells

EXCEPT sex) Ex: red blood, skin, muscle

2. Sex Otherwise known as gametes Ex: sperm, egg

Genome

The cell's hereditary endowment of DNA

Usually packaged into chromosomes (easier to manage during the cell cycle)

Prokaryotes: Single stranded, in cytoplasm, circular in shape

Eukaryotes: Double stranded, in nucleus, helix/spiral in shape

Chromosomes

Made of a DNA and protein complex called chromatin

During cell division, the chromatin becomes highly condensed into the chromosomes

# of chromosomes: Sex cell (haploid cell)▪ 23 (no pairs)—only contains one set of

chromosomes Somatic cell (diploid cell)▪ 46 (in 23 pairs)—contain two sets of chromosomes

Chromosomes vs. Chromatin

Chromatin Long, thin fiber Uncondensed Before/after cell division Genetic material usually in this form

Chromosome Characteristic “X” shape Condensed into this shape Only during cell division

Chromosomes - Structure

At cell division, each chromosome has been duplicated

The duplicated chromosome consists of two sister chromatids

Centromere – the point where two sister chromatids are connected

Cell Cycle – 2 parts

1. Interphase - (90% of cycle) - when the cell grows and duplicates the chromosomes

2. Mitotic Phase (M) - when the chromosomes are split into separate cells

Interphase - parts

In order: G1 - first gap S - synthesis G2 - second gap

G1, S, G2

G1: Cell grows and carries out regular

biochemical functionsS:

DNA is replicated or synthesized - chromosomes are replicated

G2: Cell completes preparations

Comment: A cell can complete S, but fail to enter G2.

Mitotic Phase - parts

1. Mitosis - division of replicated chromosomes

2. Cytokinesis - division of the cell’s cytoplasm

Mitosis - Purpose

To divide the 2 copies of the DNA equally

To separate the sister chromatids into separate cells

Mitosis Steps

1. Prophase2. Prometaphase3. Metaphase4. Anaphase5. Telophase

Mitosis Video #

1Mitosis Video #

2

Prophase

Prophase

Nucleoli disappearChromatin condenses into

chromosomes Centrioles separate to

opposite ends of the cellMitotic spindle begins to form

Prometaphase

Prometaphase

Nuclear envelope dissolvesSpindle fibers join with the kinetochore of the centromeres

Metaphase

Metaphase

Centrioles now at opposite ends of the cell

Chromosomes line up on the metaphase plate

Spindle apparatus fully developed

Anaphase

Anaphase

Centromeres break and the duplicate chromosomes are pulled away from each other toward opposite ends of the cell

Cell elongates; poles move slightly further apart

Kinetochores

Specialized regions of the centromeres where spindle microtubules attach

Appear to “ratchet” the chromosome down the spindle fiber microtubule using a motor protein

Microtubules dissolve behind the kinetochore

Telophase

Telophase

Chromosomes uncoil back to chromatin

Nuclear envelope reformsNucleoli reappearSpindle fibers disappearCytokinesis usually starts

Cytokinesis

Cytokinesis – Animal vs. Plant

Animal Cleavage furrow forms Microfilaments contracts and divides the

cytoplasm into two partsPlant

Cell plate develops in between the two new daughter cells (from Golgi vesicles)

New cell wall developed around the cell plate

Animal Cell - Mitosis

Plant Cell - Mitosis

Evolution of Mitosis

Evolution, cont.

Hypothesis: Mitosis has origins in prokaryotic cells How do we know this?▪ Proteins involved are identical▪ Ex: kinetochores, protein kinase

checkpoints, etc.

Regulation of Cell Division

Must be controlledRate of cell division depends on

the cell type Ex - skin: frequently liver - as needed brain - rarely or never

Checkpoints

A critical control point in the cell cycle

Several are knownCells must receive a “go-ahead”

signal before proceeding to the next phase

G1 Checkpoint

Also called the “restriction point” in mammalian cells

Places cells in a non-dividing phase called the Go phase

GO

Go Phase

Non-dividing stateMost cells are in this stateSome cells can be reactivated

back into M phase from the Go

phase

Protein Kinase Checkpoint - G2

Uses protein kinases to signal “go-ahead” for the G2 phase

Activated by a protein complex whose concentration changes over the cell cycle

GO

Protein Kinase

MPF

M-phase Promoting FactorProtein complex required for a

cell to progress from G2 to Mitosis

Role of MPF - to trigger a chain of protein kinase activations

Active MPF has: cdk and cyclin

GO

Protein KinaseMPF

CDK and Cyclin

CDK: Protein Kinase Amount remains constant during cycle Inactive unless bound with cyclin

Cyclin: Protein whose concentration builds up

over G1, S and G2 When enough cyclin is present, active

MPF is formed

Active MPF

Triggers MitosisActivates a cyclin-degrading

enzyme, which lowers the amount of cyclin in the cell

Result - no active MPF to trigger another mitosis until the cycle is repeated

Growth Factors

External signals that affect mitosis

Examples: PDGF Density-dependent inhibition Anchorage dependence

PDGF

Platelet-Derived Growth Factor

Stimulates cell division to heal injuries

Density-Dependent Inhibition

The number of cells in an area force competition for nutrients, space, and growth factors

When density is high - no cell division

When density is low - cells divide

Anchorage Dependence

Inhibition of cell division unless the cell is attached to a substratum

Prevents cells from dividing and floating off in the body

Cancer Cells

Do not stop dividing. The control mechanisms for

cell division have failed

Cell division balance

Regulation of cell division is a balance between:

Mitosis - making new cells Apoptosis - cell suicideCancer can result if either

process doesn’t work

Summary

Identify the roles of cell division Identify the composition of a chromosome. Recognize the phases of the cell cycle. Identify the stages and characteristics of

Interphase. Identify the stages and characteristics of

Mitosis. Recognize the mechanisms of Cytokinesis. Recognize factors that control cell division. Recognize the results when the regulation of

cell division goes wrong.