CELL DIVISION MITOSIS AND CYTOKINESIS - TSFX – · PDF fileCELL DIVISION MITOSIS AND CYTOKINESIS ... MITOSIS & MEIOSIS SUMMARY Feature Mitosis Meiosis Site of division All somatic

The School For Excellence 2011 The Essentials – Biology – Book 1 Page 26

CELL DIVISION

MITOSIS AND CYTOKINESIS Mitosis involves the division of one cell into two identical daughter cells, i.e. each daughter cell contains the same number of chromosomes as the original cell. Strictly speaking, mitosis is the nuclear division of the cell, and cytokinesis is the process of cytoplasmic division. Mitosis occurs in the somatic cells of organisms. It is the process by which growth and

repair of tissues occurs in animals, and growth (eg. in root and shoot tips) occurs in plants. It is also a process by which asexual reproduction occurs (eg. budding in yeast and hydra, use of runners in some plants).

It is the reproductive process that occurs in tissue culture, skin grafts and cloning

organisms.

THE FOUR PHASES OF MITOSIS IN A CELL (2n = 4) PROPHASE Chromosomes shorten and thicken (condense). In animal cells, centrioles move to the poles. The nucleus disappears, chromosomes continue to shorten and thicken, and chromatids becoming distinct. Spindle fibres begin to form. Chromosomes attach to spindle fibres.

METAPHASE Chromosomes are arranged along the equator (middle) of the cell. Each chromosome is attached to a spindle fibre by the centromere.


ANAPHASE The chromatids split (perform disjunction) at the centromere and daughter chromosomes migrate to either end of the cell; spindle fibres begin to disappear. TELOPHASE The cytoplasm begins to constrict and the cell starts to pinch in half (in animal cells). Cell plate forms in plant cells. Spindle fibres disappear. The nucleus reappears and the chromosomes are no longer visible.

Kinetochores are protein structures located on chromosomes, and are the sites where spindle fibres attach during division to in order to pull the chromosomes apart. CYTOKINESIS Cytokinesis (cytoplasmic division) follows telophase. Cytokinesis is not part of mitosis. Daughter cells grow to reach their regular size during this period. In animal cells, a cleavage furrow forms between dividing cells at cytokinesis. In plant cells, a cell plate forms from materials supplied by golgi vesicles. This will

ultimately form the cell wall.

Mitosis ensures that cells maintain a large surface area to volume ratio. It allows for: 1. Growth of an organism. 2. Replacement of cells which have died or been damaged.


MITOSIS SUMMARY

(Roberts MBV, 1987)

Interphase is not part of mitosis


ASEXUAL REPRODUCTION Asexual reproduction does not involve the fusion of two gametes (sex cells). eg: binary fission in bacteria, plant division using cuttings, bulbs etc. The alleles (genotypes) of the parent and the offspring are identical. It still requires some type of DNA replication.

BINARY FISSION Binary fission is a type of cell division by which prokaryotes reproduce. The single chromosome replicates prior to cell division, so that each daughter cell receives a single copy of the parent cell chromosome.

SEXUAL REPRODUCTION Sexual reproduction involves the fusion of gametes. Gametes are typically produced in a process of cell division called meiosis. The term gamete refers to either an egg or sperm. Gametes are haploid (n), i.e. they do not contain chromosomes in pairs. They only

contain one set of chromosomes (and therefore genes) in their nuclei. Hence, gametes contain half the number of chromosomes as somatic (diploid) cells. In humans, somatic cells contain 46 chromosomes, and gametes 23 chromosomes.

Hence, when the egg and sperm join during fertilisation, the newly formed cell will have

the diploid number of chromosomes restored, enabling the cell to function.

(power-chemicals.com)


MEIOSIS AND CYTOKINESIS Meiosis is a reduction division involving one diploid parent cell dividing to produce four haploid daughter cells, i.e. each daughter cell has half the number of chromosomes of the parent cell.

Meiosis involves the random assortment of chromosomes into their homologous pairs. A dramatic increase in variation within a species results as gametes from the one individual are not all identical. The daughter cells are known as gametes (sex cells). Each gamete has a single copy

of each type of chromosome. All cells involved in the process of meiosis, including the gametes, are known as

germline cells. Meiosis takes place in sex organs (gonads). The gonads of male organisms are known as ________________________________ The gonads of female organisms are known as _______________________________ A human somatic cell contains 46 chromosomes. There are in 23 pairs of homologous

chromosomes in females, and 22 pairs (as well as an X and Y chromosome) in males. As mentioned earlier, homologous chromosomes carry the same genes at

corresponding loci, however, they needn’t carry the same alleles for those genes.

eg: If a gene is coding for height, the allele on one of the pair of chromosomes could be coding for tallness while the other allele on the other chromosome could be coding for shortness. During meiosis in humans, a diploid cell containing 46 chromosomes divides twice to

produce four gametes, each containing 23 chromosomes. When two gametes fuse during fertilisation, a zygote is formed, containing the full diploid set of chromosomes.

Meiosis involves two divisions (PMAT I and PMAT II). The first division is the

reduction division, as the number of chromosomes per cell has been halved after this division.

stash.norml.org


INTERPHASE (PRIOR TO MEIOSIS) As with mitosis, interphase occurs prior to meiosis. DNA replication occurs once only prior to the two divisions in meiosis, hence, four haploid daughter cells will be created.

PROPHASE I Nucleus disappears, chromosomes shorten and thicken and become visible. Homologous chromosomes pair (synapsis) and spindle fibres form. Chromosomes attach to spindle fibres. Crossing over (see page 32), which is a very important source of genetic variation in the gametes, can take place at this time.

METAPHASE I Spindle fibre formation complete, homologous chromosomes line up in pairs along equator of cell. Independent Assortment of Chromosomes (see page 34): Maternal and paternal chromosomes of each homologous pair line up independently of all other homologous pairs. This is a very important source of genetic variation within the species!!

2

Note: You will never observe this situation during metaphase of mitosis.


ANAPHASE I Each homologous pair separates (performs disjunction) as double-stranded chromosomes move to opposite poles. Spindle fibres begin to disappear.

TELOPHASE I AND CYTOKINESIS Spindle breaks down, nucleus reforms, cell pinches into two daughter cells each containing half the original number of chromosomes (however, each chromosome still consists of two chromatids).

A resting phase for both cells occurs in many species. No duplication of chromosomes takes place before the second division. The second division of meiosis can now proceed. Meoisis II is very similar in process to mitosis.


PROPHASE II In both cells the nuclear membrane breaks down, spindle fibres form and chromosomes become distinct.

METAPHASE II In both cells chromosomes move to the centre of the cell (notice that the chromosomes are not aligned in pairs). Centromeres start to divide.

ANAPHASE II As centromeres divide, chromatids separate to form daughter chromosomes. In both cells the daughter chromosomes move to their respective poles (again performing disjunction); spindle fibres disappear.


TELOPHASE II AND CYTOKINESIS Both cells divide in two resulting in the production of four haploid daughter cells. Nucleus reforms and each cell pinches in half (cell plates appear in plant cells).

CYTOKINESIS (NOT PART OF MEIOSIS) Cytokinesis (cytoplasmic division) begins during telophase II. Remember, cytokinesis is not part of meiosis. In animal cells, a cleavage furrow forms and deepens until the cell is pinched in two. In plant cells, golgi vesicles, laden with cell wall materials, fuse to form a bigger and bigger cell plate, until the plate’s membrane fuses at its edges with the cell membrane, dividing the cell in two. Contents of the cell plate form the new cell wall. In either case, four haploid daughter cells have formed.


IMPORTANT SOURCES OF VARIATION DURING MEIOSIS

1. CROSSING OVER AT PROPHASE I When homologous chromosomes pair during Prophase I, they often make contact and become intertwined. During this time, a section of the genetic material on a maternal chromatid can be swapped for the corresponding material on the homologous paternal chromatid. The point of crossing over is called the chiasma (plural = chiasmata). This can lead to new combinations of genetic material on a chromosome, referred to as recombination. The crossover value between two genes located on the same chromosome can be determined by the formula:

The number of recombinant offspring (due to crossing over) x 100

Total number of offspring

eg: If crossing over occurs during 4% of meiotic divisions between two particular genes, the two genes are said to be four map units apart. The further apart two gene loci are on the same chromosome, the greater the chance of crossing over occurring between the two genes. eg: In the diagram below, crossing over would occur more frequently between genes A and C than between genes A and B or B and C.

http://www.evolutionpages.com/homo_pan_divergence.htm

Crossing over and recombination during meiosis


CHROMOSOME MAPS A chromosome map is a diagram showing some of the gene loci on a chromosome. Using the crossing over frequency (crossover value), you can determine the relative position of genes on a chromosome and create a chromosome map. The amount of crossing over is a direct link to the distance between genes on a

chromosome. Two distant genes on a chromosome will have a higher rate of crossing over than two genes that are close together.

A crossover value of 1% = 1 genetic map unit. eg: If genes A and C have an 18% crossover value, genes B and C have a 3%

crossover value and genes B and A have a 15% crossover value, a chromosome map for these genes can be drawn in the following manner:

Remember, the crossover value between two genes can be determined by the formula:

The number of recombinant offspring (due to crossing over) x 100

Total number of offspring

A B C

18 mu 18 mu

15 mu 3 mu


2. INDEPENDENT ASSORTMENT OF CHROMOSOMES AT METAPHASE I

Pairs of homologous chromosomes assort independently of all other pairs at Metaphase I, resulting in significant gamete variation.

eg: There are (2)23 different gametes possible from a human as a consequence of independent assortment!

An organism with a diploid no. of 2n = 6, can produce 23 genetically different possible types of gametes as a consequence of independent (random) assortment of chromosomes at Metaphase I:

Parent cell

Gametes


MITOSIS & MEIOSIS SUMMARY

Feature Mitosis Meiosis

Site of division

All somatic cells except brain and nervous tissue.

No. of divisions 2

No. of daughter cells 2

No. of chromosomes / daughter cells

Types of daughter cells Somatic cells.

Separation of chromatids

At anaphase.


RNA (RIBONUCLEIC ACID) Ribonucleic acid (RNA) is a polymer of nucleotides. The nucleotides are composed of the sugar ribose, a phosphate unit and one of four nitrogenous bases. The nucleotides in the single nucleotide strand of an RNA molecule are very similar in structure to DNA nucleotides: Label the following RNA nucleotide:

An RNA base can be one of four types:

1. Adenine

2. Uracil

3. Guanine

4. Cytosine 3 forms of RNA exist: Messenger RNA (mRNA) ____________________________________________ ____________________________________________ Transfer RNA (tRNA) ____________________________________________ ____________________________________________ Ribosomal RNA (rRNA) ____________________________________________ ____________________________________________

Complementary base pairing can exist within the RNA molecule (eg. within a viroid, a tRNA or rRNA molecule or within retroviral RNA), between the RNA molecule and a DNA molecule, or between two different RNA molecules (eg. between tRNA and mRNA during protein synthesis).

When bases meet, Uracil pairs with Adenine and Guanine pairs with Cytosine.

A

B

C


eg: Complete the following tables:

DNA A A C G C C T A T T A C G

RNA

RNA U G G C G A U G A A A U U

RNA

DNA VERSUS RNA

DNA RNA

No. of Nucleotide Strands 2

Sugar Deoxyribose

Bases A, T, C, G

Functional Location Nucleus


GENE ACTION

INTRONS AND EXONS The coding regions of genes contain sections known as introns and exons.

ExonsExons

Parts of the coding Parts of the coding region of a gene thatregion of a gene thatare are both transcribed both transcribed and translated and translated

IntronsIntrons

Parts of the coding Parts of the coding region of a gene thatregion of a gene thatare are transcribed transcribed but not translatedbut not translated

ExonExon 33

ExonExon 22ExonExon 11 IntronIntron 11 IntronIntron 22 ExonExon 33

ExonExon 22ExonExon 11

IntronIntron 11IntronIntron 22

ExonExon 33

Post transcription Post transcription modificationmodification

Transcription Transcription (RNA synthesis)(RNA synthesis)

Translation Translation (protein synthesis)(protein synthesis)

DNADNA

PrePre--mRNAmRNA

Messenger RNAMessenger RNA

Ribosome Ribosome

GeneGene

ExonExon 22ExonExon 11 IntronIntron 11 IntronIntron 22

Nuclear membraneNuclear membrane

AAAAAAAAMethyl capMethyl cap PolyPoly--A tailA tail


FLANKING REGIONS Flanking regions are regions of DNA on either side of the coding region of a gene. Two flanking regions exist for any gene: i. Upstream Region The flanking region is the section of DNA before the start of the coding region of a gene. It includes the promotor sequence (a section that binds RNA polymerase and indicates where to start transcribing RNA). The upstream regions have survived millions of years of evolution. If the upstream region is mutated it can affect the activity of the coding region (gene) that follows it. Hormones can attach to specific upstream regions, thereby affecting gene action. ii. Downstream Region The downstream flanking region is located after the end of the coding region of a gene. It includes an “end transcription signal” which terminates transcription. If the downstream region is mutated, gene action can be altered, so this region also plays a role in transcription.

Promotor/Upstream region

Start Coding region Stop

Downstream region

Promotors The promoter is a control region of the upstream region that exists to switch the gene

on or off. Promoters are non coding base sequences that signal the start of a gene. The promoter commonly includes a TATA box. RNA polymerase binds to the promoter region, thus activating the gene and initiating transcription.

The promoter of gene A may be repressed by a protein produced by another regulatory gene, gene B. The repressor protein product of gene B turns off gene A.

In prokaryotes: A prokaryote promoter has two essential sequences. One is the sequence recognised by RNA polymerase, to which it binds. The second, closer to the initiation site, is the TATA box where the DNA begins to separate (dissociate) so that the template strand can be exposed.

In eukaryotes: Eukaryote RNA polymerase cannot simply bind to the promoter and initiate transcription. In eukaryotes a collection of regulatory proteins called transcription factors mediates the binding of RNA polymerase and the initiation of transcription. A protein binds to the TATA box then other regulatory proteins, called transcription factors bind to that protein and each other. Transcription complexes form, then RNA polymerase can bind. Once the polymerase is firmly attached to the promoter DNA the two DNA strands unwind there and the enzyme starts transcribing the template strand.

National Geographic Society


GENE EXPRESSION

THREE BASE SEQUENCE TERMINOLOGY A three base sequence of DNA coding for an amino acid is known as a triplet. A three base sequence of mRNA coding for an amino acid is known as a codon. A three base sequence of tRNA coding for an amino acid is known as an anticodon.


TRANSCRIPTION When a gene becomes active or is switched on in a eukaryotic organism, it must make a mobile copy of the genetic instructions to be received by the ribosomes. This is because DNA (instructions) cannot move out of the nucleus. The DNA is copied (transcribed) into mRNA. Transcription is the synthesis of RNA from a DNA template. It occurs in the nucleus of eukaryotic cells and the cytoplasm of prokaryotic cells. Step 1: The enzyme RNA polymerase attaches to the promoter region of DNA just upstream of the gene. Step 2: The double stranded DNA making up this gene is unwound by the RNA polymerase by the breaking of the weak hydrogen bonds existing between its two strands. Unpaired bases on the template strand are now exposed. Step 3: RNA polymerase constructs mRNA by collecting free complementary RNA nucleotides to the exposed DNA template strand. It assembles the RNA nucleotides according to complementary base pairing rules. The nucleotides in eukaryotic cells are joined to form a single stranded pre-mRNA molecule. Pre-mRNA contains both introns (non-coding “interruption” nucleotide sections) and exons (“expressed” nucleotide sections). The DNA molecule reforms a double helix. The RNA transcript is assembled in the 5’ to 3’ direction.


POST-TRANSCRIPTION MODIFICATION

Step 4: Enzymes cut out the introns or non-coding regions from the pre-mRNA. The remaining exons are joined together. A methyl cap is added to one end of the molecule, and a poly-A tail at the other. The resulting shorter mRNA molecule leaves the nucleus via a nuclear pore to perform its function at the ribosomes.

Example: DNA template: A T G C C T G A A T The mRNA copy is: U A C G G A C U U A As you can see the Thymine (T) in DNA is replaced with Uracil (U) in RNA. Complete the following table:

DNA antisense strand

DNA template strand

T A C G C C G T A A C C G G T G C A

mRNA

The human genome contains 20,000 – 25,000 genes, however, there may be as many

as 400,000 proteins translated from these genes. Originally it was believed that one gene coded for one protein, but obviously the maths does not work!

Research now shows that genes are regulated in different ways and that one gene can

in fact code for more than one protein. The above differences may be due to differences in intron retention, i.e. differing

amounts and types of introns are cut out from pre mRNA in different situations, resulting in the massive variety of proteins from a much smaller number of genes.

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CELL DIVISION MITOSIS AND CYTOKINESIS - TSFX – · PDF fileCELL DIVISION MITOSIS AND CYTOKINESIS ... MITOSIS & MEIOSIS SUMMARY Feature Mitosis Meiosis Site of division All somatic