ICSE 10>BASIC BIOLOGY>1. CELL DIVISION

Scope of syllabus

Cell Division: Mitosis and its stages. 
  • A basic understanding of Meiosis as a reduction division (stages not required). 
  • Significance and major differences between mitotic and meiotic division.
  • Basic structure of chromosome with elementary understanding of terms such as chromatin, chromatid, gene.
  • Structure of DNA and centromere.

CLASS PRESENTATION

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Helpful links

CHECK OUT SOME ANIMATIONS
http://www.cellsalive.com/mitosis.htm
http://www.maxanim.com/genetics/Mitosis/Mitosis.htm
http://www.sumanasinc.com/webcontent/animations/content/mitosis.html
QUIZ ON CELL DIVISION
http://www.syvum.com/cgi/online/oatm.cgi/squizzes/biology/mitosis.tdf?0
http://online.santarosa.edu/testbank/?11321 
What is cell division?
Cell division is a method by which new cells arises from pre-existing cells.

Observe the given picture of a cut finger.
Healing process is producing new cells.
The new cells produced should be exactly the same as their parent cell.
http://schremppstudio.com/cutfinger3.gif
http://universe-review.ca/I10-04-cellnucleus.jpg
Chromosomes in the nucleus of the cell are the carriers of genetic information. 
Every organism has a specific chromosome number.
The number of these chromosomes should remain same inorder to maintain the cell structure and function.
Thus when a cell divides, the daughter cells should receive the same chromosome number.
Before beginning with the process of cell division let us get familiar with the chromosome structure.

CHROMATIN AND CHROMOSOME

Nucleus contains thread like structures- chromatin
http://www.biology4kids.com/files/art/cell_nucleus2.gif
CHROMATIN
Chromatin is a mass of genetic material composed of DNA and proteins that condense to form chromosomes. 
CHROMOSOME
Chromosome is an organized structure of DNA and protein that is found in cells. 
CHROMATID
Chromatid is the daughter strand of a duplicated chromosome that is joined by a single centromere. 
http://online.santarosa.edu/homepage/cgalt/BIO10-Stuff/Ch08-A-Mitosis/Chromatin-VS-Chromosomes.JPG
http://online.santarosa.edu/homepage/cgalt/BIO10-Stuff/Ch08-A-Mitosis/Chromosome-VS-Chromatid.JPG

Chromatid and DNA

Chromatid has a highly coiled of DNA and proteins. This thread like structure is called chromonema.
The DNA is coiled on proteins called histones. 
DNA coils around a core of eight histones forming a complex. 
This complex is called as nucleosome.

Where is the DNA?

Structure of DNA

Discovery of DNA

Picture
James Watson and Francis Crick  discovered the structure of DNA and were awarded the Nobel prize in 1962.
They constructed a double helix structure in 1953
DNA is the hereditary material present in humans and almost all organisms. (some viruses have RNA as genetic material)
Most DNA is present in the nucleus (nuclear DNA) and some DNA can be found in the mitochondria (mitochondrial DNA).
http://evolution.berkeley.edu/evolibrary/images/history/dna_structure.gif
DNA is made up of two long parallel strands that are helically coiled around an axis.
The strands run in the opposite direction and is made up of repeating units called nucleotides.
Picture
Structure of a nucleotide
Nucleotide has three sub units- 
  1. Pentose sugar- Deoxyribose sugar
  2. Phosphate group
  3. Nitogenous base- It is of two type- Purines (Adenine and Guanine) and Pyrimidines (Cytosine and Thymine)
DOUBLE HELIX STRUCTURE-
  • The bases are bonded with sugar molecules. 
  • The sugar molecules are connected by phosphate bonds, thus it forms a phosphate-sugar-phosphate-sugar backbone. 
  • The two chains of the DNA are held together by hydrogen bonds. 
  • These bonds occur between complementary nitrogenous bases. 
  • The base pairing is always between A - T  and G-C. 
  • There are two hydrogen bonds between A-T and three between G-C.
DRAW THE DIAGRAM
http://newsimg.bbc.co.uk/media/images/40060000/gif/_40060699_dna_bbc_203.gif

DNA and chromosome

Picture
The DNA is packed with proteins to form chromosome.

Humans have 46 chromosomes. (23 pairs)

DID YOU KNOW
The record for minimum number of chromosomes belongs to a subspecies of the ant Myrmecia pilosula, in which females have a single pair of chromosomes.

The record for maximum number of chromosomes is found in found in the fern family.  Ophioglossum reticulatum  This fern has roughly 630 pairs of chromosomes or 1260 chromosomes per cell. 

HOMOLOGOUS CHROMOSOME AND SISTER CHROMATID

http://www.phschool.com/science/biology_place/labbench/lab3/images/homologs.gif
HOMOLOGOUS CHROMOSOME-
These are chromosome pairs of approximately the same length, and centromere position.
Out of the pair one is inherited from the mother (maternal) and one from the father (paternal).

SISTER CHROMATID-
These are two identical chromatids connected by a centromere.

Chromosome Classification

http://2.bp.blogspot.com/_fbXoeoCa5FQ/TGxfoW-eqxI/AAAAAAAAABk/OP8TaiYSEN0/s1600/16694_Types+of+chromosomes.JPG
Based on the position of the centromere, the chromosomes can be classified into four types 
  • The metacentric chromosome has middle centromere forming two equal arms of the chromosome. 
  • The sub-metacentric chromosome has centromere nearer to one end of the chromosome resulting into one shorter arm and one longer arm. 
  • In acrocentric chromosome the centromere is situated close to its end forming one extremely short and one very long arm.
  • The telocentric chromosome has a terminal centromere. 
FUNCTION OF CHROMOSOME- carry genetic information from one generation to the other. 

KARYOTYPE
When the chromosomes of a person are arranged in an organised manner it is called a Karyotype. 
Here the chromosomes are arranged and numbered by size, from largest to smallest. 
This arrangement helps scientists quickly identify chromosomal alterations that may result in a genetic disorder. 

Diploid and haploid

A cell having a single set of paired chromosomes is Diploid
http://course1.winona.edu/sberg/IMAGES/human_karyotype2.jpg
A cell having a single set of unpaired chromosome is haploid
http://course1.winona.edu/sberg/IMAGES/human_karyotype3.jpg
THE CELL CYCLE
Picture
http://virtuallaboratory.colorado.edu/BioFun-Support/AllGraphics/eukCycle.gif

G-1 post mitotic phase 

  • Cell absorbs nutrients and grows larger. 
  • RNA and proteins are synthesized 
  • No change in DNA content. 

S phase

  • Synthesis of new DNA (DNA replication) 

G-2 pre mitotic phase

  • Cell continues to grow
  • Protein synthesis for asters and spindle fibres 
  • No DNA synthesis.

CELL DIVISION

Cell divides giving rise to two daughter cells that are identical to the parent cell and has the same chromosome number.
MITOSIS
produces diploid cells
http://www.daviddarling.info/images/cell_division.jpg
Cell divides giving rise to four daughter cells with half the number of chromosomes.
MEIOSIS
produces haploid cells
Sometimes there is direct cell division by simple cleavage of the nucleus, without spindle formation or the appearance of chromosomes. 

This type of division is seen in prokaryotic cells and amoeba.
This is called AMITOSIS.
http://images.tutorvista.com/content/reproduction-in-animals/simple-binary-fission-amoeba.jpeg

Mitosis is also called- Equational division.

Picture
  • Mitosis is an equational division in which a single cell divides to form two daughter cells.
  • The daughter cells are identical to each other and the parent cell in all respect.
  • Mitosis maintains the chromosome number i.e, the daughter cells have the same number of chromosome as the parent.
INTERPHASE
Growth period between two successive divisions.
  • Synthesis of new genetic material.
  • Increase in volume of cytoplasm and nucleus
Interphase was earlier called as "resting phase" do you think this term is appropriate?
PROPHASE
  • Chromatin material condenses to form thread-like chromosomes.
  • Centrioles start to migrate to the opposite poles of the cell.
  • Two star like structures are formed at the poles from the radiating fibers.
  • Spindle fibers are formed between the two centrioles
METAPHASE
  • Chromosome shorten and condense further.
  • Chromosome arrange at the equator, centromeres at the equator and arms face the poles.
  • Spindle fibers attach to the kinetochores on the centromere.
The spindle apparatus, is a network of microtubules (also called "spindle fibers") that forms within a dividing eukaryotic cell, both during mitosis and meiosis 
The kinetochore is the protein structure on chromatids where the spindle fibers attach during cell division to pull sister chromatids apart.
Metaphase plate is a plane in the equatorial region of the spindle in dividing cells, along which the chromosomes become arranged during the metaphase.
Kinetochore microtubules are spindle fibers that attach to the kinetochores and move the chromosomes to the center of the cell. 
Polar microtubules are not attached to chromosomes but overlap each other. 
Asters are short microtubules that radiate from the centrosomes. 
ANAPHASE
  • Centromere divides such that two chromatids has its own centromere.
  • The separated sister chromatids begin to move towards the opposite poles due to the contraction of spindle fibers.
  • At the end of anaphase the chromosomes reach  the opposite poles.
TELOPHASE
  • Chromatids uncoil and change to chromatin network.
  • The nuclear membrane reappears.
  • The spindle fibers disappear and centrosome organises above the nucleus.

Stages of mitosis as seen under microscope

In plant cell

It occurs in the region of meristems.
Centrioles are not involved.
Cytokinesis occurs by plate formation.
cell plate or middle lamellae cements the daughter cells.

In animal cell

It occurs at several places.
Centrioles are involved in division.
Cytokinesis occurs by cleavage.
Cleavage creates intercellular space between daughter cells

CYTOKINESIS

IN PLANT CELL
http://www.phschool.com/science/biology_place/biocoach/images/mitosisisg/mitcytpl.gif
  • takes place by plate formation 
  • the cell plate grows centrifugally 
  • Cell plate cements the daughter cells  
IN ANIMAL CELL
http://images.tutorvista.com/content/feed/tvcs/cytokin.gif
  • takes place by cleavage formation 
  • the cleavage occurs centripetally 
  • Cleavage creates intercellular spaces between the daughter cells
What is the need for mitosis?
  1. Growth
  2. Replacement of worn out cells
  3. Repair
  4. Reproduction

MEIOSIS is called the reductional division
FEATURES OF MEIOSIS-
Meiosis is a reductional division- chromosome number is reduced to half.
Pairing of homologous chromosomes- Synapsis
Chiasmata formation- Points at which non sister chromatids attach during synapsis
Crossing over- Exchange of genetic material between non sister chromatids.

Significance of meiosis-
Meiosis maintains definite and constant number of chromosomes in organisms.
Meiosis results in variation due to crossing over (exchange of genetic material)

Comparing mitosis and meiosis

Picture
  • Occurs in somatic cells 
  • produces two daughter cells
  • completes in one division
  • genetic constitution remains same in the daughter cell
  • it takes place during growth, repair, regeneration and development
  • occurs in reproductive cells
  • produces four daughter cells
  • completes in two divisions
  • genetic constitution is different in the daughter cells
  • it takes place during formation of gametes
sources of mitosis light micrographs-
http://www.anselm.edu/homepage/jpitocch/genbio/slidesother/inter92603.jpg
http://www.anselm.edu/homepage/jpitocch/genbio/slidesother/pro292603.jpg
http://jacusers.johnabbott.qc.ca/~biology/NYA/labs/NYALAB4/onionanaaphase.jpg
http://student.ccbcmd.edu/~gkaiser/biotutorials/dna/mitosis/images/metaphase_sa1_pc.jpg
http://schoolworkhelper.net/wp-content/uploads/2011/02/onion-mitosis-3-lab.jpg 
http://www.anselm.edu/homepage/jpitocch/genbio/slidesother/telo292603.jpg 
http://www.phschool.com/science/biology_place/labbench/lab3/images/anifield.gif