Cell division

Cell DivisionCell DivisionMitosis- Cell reproductionMitosis- Cell reproduction

Learning Objectives

• The cell cycle and its phases.

• The cell division “mitosis”.

• Cell cycle control system.

• The different abnormalities of cell cycle (cancer).

• DNA structure and DNA replication .

Key Words• Chromosome• chromatin• centriole & centrosome• centromere & kinetochore• chromatid

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chromosome

A chromosome is a long, continuous piece of DNA which contains the genetic information for a cell.

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chromatin

Chromatin is the building block for a chromosome. It consists of DNA and proteins.

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Centriole & centrosome

A centriole is a barrel microtubule structure. Two centrioles arrange themselves perpendicularly to form a centrosome.

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centromere & kinetochore• The centromere is constricted region of the

chromosomes that holds the sister chromatids together, the site where kinetochore forms.

• The kinetochore is a complex protein containing structure to which microtubules attach.

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chromatid Is a one strand of the chromosome after it has duplicated for cellular division. The two chromatids are called “sister chromatids” and will eventually split and move to opposite ends of the cell.

Sister chromatids

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Ready?Let’s begin

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Cell division

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Why do cells divide?

•Growth

•Repair and replace dead cells

•asexual reproduction

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THE CELL CYCLE______________________

_

• Define cell cycle: the cell cycle is the sequence of growth and division of a cell. It consists of three major phases:

o interphaseo mitosiso cytokinesis.

• Cell division results in two cells that are both identical to the parent cell.

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Interphase

Estimated (90% of cycle) 3 phases

•G1 phase

•S phase

•G2 phase

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• Growth of cell.

• Dupliction of organelles.

• Synthisis of proteins.

G1 phase

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• DNA synthesis occurs• DNA replication results in

duplicated chromosomes• centrosome cycle

S phase

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• Cell continues to grow and if a problem

occurs in DNA replication, it will be repaired.• Cell will prepare for mitosis. • cell synthesizes proteins needed for cell

division

G2 phase

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Estimated (10% of cycle) Includes 2 parts :

1)Mitosis•Prophase•Promataphase

•Metaphase

•Anaphase

•Telophase

2) Cytokinesis

M phase

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Mitosis

• is the process in which a eukaryotic cell separates the chromosomes in its cell nucleus, into two identical sets in two daughter cells.

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Prophase

Duration (15 min)

• Chromosomes condense (get thicker) and coil, they become visible under light microscope.

• The two sister chromatids of each chromosomes attach at a point called centromere.

• Spindle fibers begin to form from two centrosome, and they will start moving apart.

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Duration (15 min)

• The nuclear membrane disappears.

• Kinetochores will get attached to the centromere.

• The spindle fibers become attached to the kinetochore.

Prometaphase

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Duration (20 min)

• Chromosomes reach their most highly condensed state.

• The spindle fibers begin to contract to the centromeres of the chromosomes, which are now arranged along the middle of the spindle.

Metaphase

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Duration (3 min)• The centromere of each

chromosome splits, allowing the sister chromatids to separate.

• The chromatids are then pulled by the spindle fibers toward opposite sides of the cell.

• The two sets of chromosomes are identical.

• Cleavage furrow starts to form.

Anaphase:

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Duration (10 min)• New nuclear membranes are formed

around each of the two sets of 46 chromosomes.

• The spindle fibers disappear.

• Chromosomes become thinner.

• Cytoplasm starts dividing by contractile ring.

• At the end, we will have two diploid daughter cells, which are identical.

Telophase:

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Cytokinesis

• the division of the cytoplasm and organelles

• Begin in anaphase and completed by the end of telophase .

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Summary

• Interphase (90% of cycle) o G1 phase~ growth o S phase~ synthesis of DNA o G2 phase~ preparation for

cell division• Mitotic phase (10% of cycle)o Mitosis~ nuclear divisiono Cytokinesis~ cytoplasm

division

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Cell cycle control system

Cell cycle control system Involves

•Cell cycle checkpoints

•Specialized protein complex

Function control the order and timing of the cell cycle.

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( control points ) They are pathways that:•Ensure that events are completed correctly before letting the cell enter the next phase.

The checkpoint receive signals from :

inside and outside the cell

Checkpoints

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There are 3 major checkpoints :I.at the end of G1 it is large & strong enough to continue with the cell division process

II.at the end of G2

III. during mitosis phase it ensures if the spindles attached to the kinetochore within each chromosome

Checkpoints

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Is a combination of a Cycline & Cdk*

to preform its work ( help the cell to enter the next phase) it has to be activated by phosphorylated at one site and dephosphorylated at the other site. * Cycline dependent kinase

Specialized protein complex

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Now we’ll see how much you’ve learned

so raise your handand wait your turn...

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Cell Cycle

MITOSIS

interphase

prophase

metaphase

anaphase

telophase

cytokinesis

Chromosomes invisible; DNA replicates

Chromosomes appear, nucleus disappears

Chromatids pulled to poles

Cytoplasmic division

Chromosomes at equator, spindle forms

Chromatids at poles, nucleus reforms

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Cell Increase and Decrease

• Cell division:oMitosis oCytokinesis

• Apoptosis (cell death) decreases the number

of cells.• Both cell increase and apoptosis occur

during normal development and growth.

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Abnormal Division

• If there is a damage in DNA during G1 and the P53 is not present, the cell can’t stop entering into the next phase so, it will complete the cycle in an abnormal way.Then the cycle will be uncontrolled .

• If a cell has extra protein BCL2 then it can absorb all of the death signals so the cell will prevent death(apoptosis) .That will leads to a high rate of mutation and the production of cells that tend to become cancerous ( tumor ) .

Abnormalities

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http://www.youtube.com/watch?v=vKIRWY-LMYc

• The uncontrolled growth of cells in human to the

disease called cancer.

http://www.youtube.com/watch?v=vKIRWY-LMYc

• The uncontrolled growth of cells in human to the

disease called cancer.

DNA

• Before:– Chromosome duplicates = 2 sister chromatids

(DNA replication )

• During:– Sister chromatids separate

• After:– 2 “daughter” cells– Genetically identical

DNA & Mitosis

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DNA structure

• Two strands coiled called a double helix.

• Center made of nitrogen bases bonded together by weak hydrogen bonds.

• Stands for deoxyribonucleic acid made up of subunits called nucleotides.

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constituents of nucleotide

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• Phosphate group

• 5-carbon sugar

• Nitrogenous base

Nucleotide made of:

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Anti-parallel Strands

•One strand of DNA goes from 5’ to 3’

•The other strand is opposite in direction going 3’ to 5’

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DNA Replication

Models of DNA replicate?“Possibilities”

In this model , one daughter molecule

contains both parental DNA strands, and the

other daughter molecule contains DNA strands of

all newly-synthesized material.

In this model two DNA double helices, both of which consist of one parental and

one new strand.

In this model the parental double helix is

broken into double-stranded DNA segments .The

segments assemble into complete DNA double helices from new and

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1. The DNA strands are separated at a specific area called origin .

2. The complementary strands are synthesized by DNA polymerase .

3. Termination : DNA polymerase goes throughout each strand in order to find errors and correct them.

Steps of DNA replication

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Base pairing During Replication

Enzymes Required for Replication• Helicase: an enzyme that catalyzes the energy-dependent

unwinding of the DNA double helix during DNA replication.

• DNA polymerase: an enzyme that catalyzes the assembly of deoxyribonucleoside triphosphates into deoxyribonucleic acid, with single-stranded DNA serving as the template. The enzyme is often found in tumor cells.

• Ligase : Joins together small newly synthesized pieces of DNA called Okazaki fragments.

• Primase : Adds an RNA primer so that DNA synthesis can begin

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Abnormal division• What happens when DNA is damaged? DNA Structure• constituents of nucleotides. DNA Replication • Models of DNA replication.• Steps• Enzymes Required for Replication.

Summary

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• Essential cell biology – third edition

• http://www.biologie.uni-hamburg.de/b-online/library/falk/cellDivision/division.htm

• http://www.phschool.com/science/biology_place/biocoach/dnarep/classical.html

References