AP Bio_Cell Cycle.pptx

How you became you

Life of a cell from the time it is first formed until its own division into two daughter cells

The Cell Cycle

Reproduce by cell division

Unicellular organisms

100 µm

(a) Reproduction. An amoeba, a single-celled eukaryote, is dividing into two cells. Each new cell will be an individual organism (LM).

Reproduce by a type of cell division known as Binary Fission

Bacterial chromosome replicates and two daughter chromosomes actively move apart

Binary Fission

Origin of replication

E. coli cell

Bacterial Chromosome Cell wall Plasma Membrane Two copies of origin Origin Origin

Chromosome replication begins. Soon thereafter, one copy of the origin moves rapidly toward the other end of the cell.

1

Replication continues. One copy of the origin is now at each end of the cell.

2

Replication finishes. The plasma membrane grows inward, and new cell wall is deposited.

3

Two daughter cells result.

Depend on cell division for three main things:

◦

Development from a fertilized cell

◦

Growth

◦

Repair

Multicellular Organisms

20 µm 200 µm

(b) Growth and development. This micrograph shows a sand dollar embryo shortly after the fertilized egg divided, forming two cells (LM).

(c) Tissue renewal. These dividing bone marrow cells (arrow) will give rise to new blood cells (LM).

Results in two genetically identical daughter cells

Before division occurs, daughter cells need to

have an exact copy of the genetic material (DNA)

DNA contains the blueprint (genome)

DNA molecules are packaged into chromosomes

◦ Eukaryotes: Chromatin {DNA + Protein (histones)}

◦ Animals: Somatic Cells have 2 sets of chromosomes

◦ Gametes: Have 1 set of chromosomes

What is a gene?

A sequence of nucleotides that provides the cell with instructions to make a RNA or a protein

◦ Average is 1000-4000 nucleotides per gene

Genes influence how cells, tissues and organs appear

What is a chromosome?

Chromosome: where cells package DNA

Chromatin: Strings of DNA and associated proteins called histones

◦ State of the DNA inside the nucleus when the cell is not dividing

How many chromosomes do you have?

Most humans have 23 pairs of chromosomes

◦ Called a homologous pairs

◦ Autosomes: 1-22

◦ Sex Chromosomes: X and Y

Chromosome Structure

Consist of two thin rod-like structures of DNA called sister chromatids

◦ Exact replicas of each other copied during DNA Replication

Before Cell Division

DNA is replicated and the chromosomes condense Each duplicated chromosome has two sister

Before Cell Division

Centromeres Sister chromatids

A eukaryotic cell has multiple chromosomes, one of which is

represented here. Before duplication, each chromosome

has a single DNA molecule.

Once duplicated, a chromosome consists of two sister chromatids connected at the centromere. Each chromatid contains a copy of the

DNA molecule.

Mechanical processes separate the sister chromatids into two

chromosomes and distribute them to two daughter cells.

Eukaryotic Cell Division consists of:

◦ Mitosis: Division of the Nucleus

◦ Cytokinesis: Division of the Cytoplasm

In Meiosis

◦ Sex cells are produced after a reduction in chromosome number

● But…that’s next semester…

Regulation system for the cell division process Consists of the Mitotic phase and Interphase

The Cell Cycle

The cell spends most of the time in this phase

“Living” phase of the cell

Three main stages:

◦

G

phase

◦

S phase

◦

G

phase

Chromosomes are indistinct

Nucleolus may be visible

Centrioles are present

The cell grows and functions normally Protein synthesis occurs

More organelles are produced

Until the cell divides it will stay in this phase

The cell duplicates its DNA

Cell resumes its growth in preparation for division Checks for any errors in the duplicated DNA

Mitosis and Cytokinesis occur

Consists of five phases:

◦ Prophase

◦ Prometaphase

◦ Metaphase

◦ Anaphase

◦ Telophase

Centrosomes

◦ Where the spindle arises from

Centrioles

◦ Responsible for organization of the microtubules that form the

spindle

Asters

◦ Star shaped system of microtubules formed around each centrosome

Kinetochores

◦ Responsible for attachment of the chromosomes and the

microtubules that are part of the spindle

Mitotic Spindle

◦ Microtubules that control chromosome movement during Mitosis

Phases of Mitosis

G₂ OF INTERPHASE PROPHASE PROMETAPHASE Centrosomes

(with centriole pairs) Chromatin (duplicated) Early mitotic spindle Aster Centromere Fragments of nuclear envelope Kinetochore Nucleolus Nuclear envelope Plasma membrane Chromosome, consisting of two sister chromatids

Kinetochore microtubule

Figure 12.6

Phases of Mitosis

Centrosome at one spindle pole

Daughter chromosomes

METAPHASE ANAPHASE TELOPHASE AND CYTOKINESIS

Some spindle microtubules attach to the kinetochores of chromosomes and move them to the metaphase plate

Mitotic spindle: A closer look

Centrosome Aster Sister chromatids Metaphase Plate Kinetochores Overlapping nonkinetochore microtubules Kinetochores microtubules Centrosome Chromosomes Microtubules 0.5 µm

1 µm

In animal cells, a process called cleavage occurs.

Cytokinesis: A Closer look

Cleavage furrow

Contractile ring of

microfilaments Daughter cells 100 µm

(a) Cleavage of an animal cell (SEM)

Frequency of division varies with the type of cell The cell cycle is regulated by a molecular control system

How do we know this?

Molecules in the cytoplasm regulate progress through the cell cycle

Experimental Evidence

In each experiment, cultured mammalian cells at two different phases of the cell cycle were induced to fuse.

When a cell in the M phase was fused with a cell in G₁, the G₁ cell immediately began mitosis— a spindle formed and chromatin condensed, even though the

chromosome had not been duplicated.

EXPERIMENT S

RESULTS

CONCLUSION The results of fusing cells at two different phases of the cell cycle suggest that molecules present in the cytoplasm of cells in the S or M phase control the progression of phases.

When a cell in the S phase was fused with a cell in G₁, the G₁ cell immediately entered the S phase—DNA was synthesized.

S

S S M M

M

G₁ G₁

Experiment 1 Experiment 2

Sequential events of the cell cycle

Checkpoints

Figure 12.14

Control system

G₂ checkpoint M checkpoint

G₁ checkpoint

G₁

S

The cell cycle stops until it receives a go-ahead signal

Red Light; Green Light

G₁ checkpoint

G₁ G₁

G₀

(a) If a cell receives a go-ahead signal at the G₁ checkpoint, the cell continues on in the cell cycle.

(b) If a cell does not receive a go-ahead signal at the G₁checkpoint, the cell exits the cell cycle and goes into G₀, a nondividing state.

Located at the end of the cell cycle’s G₁ phase

◦ Choices:

● Division

● Delay of Division

● Enter a resting stage (G₀ phase)

Located at the end of G₂ phase

Checks the success of DNA Replication

Makes the decision if the cell is ready for Mitosis

◦ Yes Goes through to the M phase

◦ No The cell stops and performs the necessary repairs

Metaphase checkpoint

◦ Is the Mitotic spindle formed properly?

◦ Are the chromosomes lined up on the plate correctly?

◦ Are the chromosomes attached to the microtubules?

● Yes Go onto Anaphase

● No Fix the problem or start the cell process of apoptosis (Cell

Death)

Cell Cycle Regulation

Late S phase: cyclins are synthesized and it

continues through G₂

Cyclin combines with Cdk to form MPF Promotes mitosis by

phosphorylating various protein

During Anaphase, the cyclin component of MPF is

Internal and External signal control the cell cycle checkpoint

Growth factors play a large part in stimulating cell division

◦ Molecular peer pressure

Normal Cells

Density-Dependent Inhibition

Anchorage Dependence

Crowded cells stop dividing

Cells need to be

attached to a surface to divide

Cells anchor to dish surface and divide (anchorage dependence).

When cells have formed a complete single layer, they stop dividing

(density-dependent inhibition).

Abnormal Cells

Cancer Cells

Do not exhibit Anchorage or Density

Dependent Inhibition

25 µm

Cancer cells do not exhibit anchorage dependence or density-dependent inhibition.

Cancer cells. Cancer cells usually continue to divide well beyond a single layer, forming a clump of overlapping cells.

(b)

Transformation: conversion of a normal cell to a cancer cell

Benign tumor: remain at the original site and do not move

Malignant tumor: have the ability to move to other locations

They do not respond normally to the body’s control mechanisms

Form Tumors

◦ Malignant tumors invade surrounding tissues and can metastasize

● Exportation cancer cells to other parts of the body where they can

form secondary tumors

Cancer Cells

Chemotherapy drugs interfere with specific steps of the cell cycle

◦ Taxol

● Freezes the mitotic spindle by preventing it from shortening

Radiation

● Destroys the DNA of cancer cells

● Most cancer cells have lost the ability to repair their DNA when it is

destroyed