3.0 CELL DIVISION (3 hours)
Chapter Outlines:
3.1 The Concept of Cell Division
The importance of cell division in living organisms.
Basic characteristics
DNA and chromosomes
1.2 The Cell Cycle
Four stages in the cell cycle
Events during the G1, S, G2 & M
The Concept of Cell Division
Basic characteristics:
Cell division results in the increase in number of cells
Cell division is the means whereby parents pass on genetic material to the daughter cells
Happens in 2 ways:
Mitosis – in somatic cells
Meiosis – in reproductive organs
Cell Division by Mitosis & Meiosis
Chromosome Condensation
Eukaryotic Chromosomal Organization
DNA – the basic building block of DNA consisting a sugar, a base, and a phosphate.
Nucleosomes – the basic structural unit of eukaryotic nuclear chromosomes, consisting of two molecules each of the four core histones.
Chromatin – The piece of DNA-protein complex that is studied and analyzed.
Chromosome – The physical structures in which the genetic material of the cell is organized.
Genetic Blueprints for Cells
FUNCTIONS OF:
DNA - codes plans for making cells
Chromosome - a single DNA molecule containing many genes
human (46 chromosomes)
Gene – each gene gives the directions for making 1 protein
humans (approximately 2000 genes in each chromosome)
Parts of chromosomes
Centromeres
region two chromosomes held are together before they are separated in mitosis
Kinetochore
proteins bind to centromere and attach chromosome to spindle in mitosis
Chromatid
one of the two visibly distinct longitudinal subunits of all replicated chromosomes that becomes visible btw early prophase and metaphase of mitosis
Sister chromatids & centromeres
When do we expect to see chromatin & chromosomes?
Chromatin
thin threads comprises DNA and associated proteins
when eukaryotic cell is not dividing (interphase)
Chromosomes
highly compacted structures from condensed chromatin.
seen at the time of cell division (prophase)
The Cell Cycle
orderly sequence of events
occurs from the time a cell is first formed
until it divides into two new cells.
Most of the cell cycle is spent in interphase.
Following interphase is the mitotic stage.
THE CELL CYCLE
The four stages of the cell cycle;
i. G1 – The first growth phase
ii. S Phase
iii. G2 – The second growth phase
iv. Mitosis
THE CELL CYCLE
i. G1 – The first growth phase
The longest phase
Volume of cytoplasm increase
Protein synthesis
Increase number of organelles: mitochondria, ER, Golgy apparatus
ii. S phase
DNA synthesis phase
The cell’s DNA replicates
iii. G2 – The second growth phase
Energy stores are increased.
This is a process of nuclear division (karyokinesis) and followed by division of cytoplasm called cytokinesis.
Review of The Eukaryotic Cell Cycle
answer this question
The diagram shows the
stages in the cell cycle.
(a)There are 40 units of DNA in a cell during stage G2. How many units of DNA would you expect to find in this cell
(i) during stage G1;
(ii) at prophase of mitosis? (2 marks)
Mitosis (1 hour)
Describe the four stages of the mitotic cell division
Explain the behavior of the chromosomes at each stage
Briefly describe the cytokinesis process
Compare the cell division in animal and plant cells
State the significance of mitosis
Definition of terms
What are sister chromatids and non-sister chromatids?
Non sister chromatids
Genetically non identical (one from maternal & another from paternal)
MITOSIS
Mitosis Process
Mitosis is divided into four distinct stages: prophase, metaphase, anaphase and telophase.
(PMAT)
What happens during mitosis?
Centromeres divide
sister chromatids separate
become daughter chromosomes.
End of mitosis, each chromosome consists of a single chromatid.
What is mitotic cell division?
is the division of somatic cells
produce cells that are an exact copy of parent cell
whereby the number of chromosomes stays constant 2n 2n.
it ensures genetic stability through generations.
PROPHASE
The chromosomes become visible, long, thin threads.
Gradually they shorten and thicken and each is seem to comprise two chromatids joined at the centromere.
From each centriole, microtubules develop and form a star-shaped structure called an aster.
Some of these microtubule, called spindle fibers, span the cell from pole to pole.
Collectively they form the spindle.
The nucleolus disappears and finally the nuclear envelope disintegrates, leaving the chromosomes within the cytoplasm of the cell.
2. METAPHASE
Centrosome positioned at opposite poles
Chromosomes move along spindle microtubules to equator of the cell
Attach to spindle fibre by means of centromere
3. ANAPHASE
Sister chromatids
Separate & move towards opposite poles
One set of chromosomes moves along the spindle microtubule to each pole of the cell.
The shorting of the spindle fibers is due to the progressive removal of the tubulin molecules of which they are made.
The energy for this process is provided by mitochondria which are observed to collect around the spindle fibers.
4. TELOPHASE
The chromatids reach their respective poles and a new nuclear envelope forms around each group. The chromatids uncoil and lengthen, thus becoming invisible again
The spindle fibers disintegrate and nucleolus reforms in each new nucleus.
CYTOKINESIS – division of cytoplasm
In Animal Cells
Occur by a process known as cleavage.
The first sign of cleavage is the appearance of cleavage furrow.
CYTOKINESIS– division of cytoplasm
In Plant Cells
Have walls but no cleavage furrow.
During telophase, vesicles derived from Golgi apparatus move along microtubules to the middle of the cell producing a cell plate.
The cell plate enlarges until its surrounding membrane fuses with the plasma membrane along the perimeter of the cell.
Two daughter cells result, each with its own plasma membrane.
A new cell wall arising from the contents of the cell plate has formed between the daughter cells.
Cytokinesis in Plants
Differences between mitosis in plant and animal cells
Significance of mitosis
Genetic stability
Mitosis produce two nuclei which have the same number of chromosomes as the parent cell.
Daughter cells are genetically identical to the parent cell and no variation in genetic information can be introduced during mitosis.
This result in genetic stability within populations of cells derived from the same parental cells.
2. Growth
The number of cell within organism increases by mitosis and this is the basis of growth in multicellular organisms.
3. Cell replacement
Replacement of cells and tissues involves mitosis.
4. Regeneration
Some animal are able to regenerate whole parts of the body, such as legs in crustacea and arms in star fish. Production of the new cells involve mitosis.
5. Asexual reproduction
Mitosis is the basis of asexual reproduction, the production of new individuals of a species by one parent organism.
Stages of Mitosis (diagrams)
Stages of Mitosis (diagrams)
Mitosis – growth & development
Asexual Reproduction by Mitosis (Protozoa)
Asexual Reproduction by Mitosis (Hydra)
Asexual Reproduction by Mitosis (yeast)
Explain the processes in Meiosis I and Meiosis II
Explain the position and changes of the chromosomes during each stage
Define chromatid, synapsis, bivalent, tetrad, chiasma, cross-over and centromere
State the significance of meiosis
Compare and contrast meiosis and mitosis
Meiosis requires two nuclear divisions and four haploid nuclei result.
Humans have 23 pairs of homologous chromosomes, or 46 chromosomes total.
Process whereby a nucleus divides by two divisions into four nuclei, each containing half the original number of chromosomes.
Consist of meiosis I and meiosis II.
The period of time between meiosis I and meiosis II is called interkinesis .
No replication of DNA occurs during interkinesis because the DNA is already duplicated.
Prophase I
The longest phase. This phase can be divided into 5 stages;
a) Leptotene
b) Zygotene
c) Pachytene
d) Diplotene
e) Diakinesis
Five Sequential Stages
Leptotene - Chromosomes condense
Zygotene - Synaptonemal complex
Pachytene - Crossing over
Diplotene - Cell growth
Diakinesis - Chromosomes recondense
Metaphase I
The bivalents become arranged around the equator of the spindle, attached by their centromeres.
Anaphase I
Spindle fibers pull homologous chromosomes, centromeres first, towards opposite poles of the spindle.
This separate the chromosomes into two haploid sets, one set at each end of the spindle.
Telophase I
The arrival of homologous chromosomes at opposite poles marks the ends of meiosis I.
Halving of chromosome number has occurred but the chromosomes are still composed of two chromatids.
Telophase I
Spindle disappear.
Cleavage furrow (animals) or cell plate (plants) then occurs as in mitosis.
Interkinesis
a period of time between two nuclear divisions of a cell where no DNA replication occur.
Meiosis II is similar to mitosis.
Prophase II
This stage is absent if interkinesis is absent.
The nucleoli and nuclear envelopes disperse and the chromatids shorten and thicken.
Centrioles, if present move to opposite poles of the cells and the end of prophase II new spindle fibers appear.
Metaphase II
Chromosomes line up separately around the equator of the spindle.
Anaphase II
The centromeres divide and the spindle fibers pull the chromatids to opposites poles, centromeres first.
Telophase II
As telophase in mitosis but in meiosis four haploid daughter cells are formed.
The chromosomes uncoiled, lengthen and become very indistinct.
The spindle fibres disappear and the centrioles replicate.
Nuclear envelope re-form around each nucleus
which now posses half the number of chromosomes of the original parents cell (haploid).
Halving the chromosome number ensures that when gametes with the haploid number fuse to form a zygote
the normal diploid number is restored.
Meiosis leads to increased variation because:
When the haploid cells fuse at fertilization there is recombination of parental genes.
During metaphase I, homologous chromosomes are together at the equator of the spindle, but they separate into daughter cells independently of each other.
Chiasmata and crossing-over can separate and rearrange genes located on the same chromosome.
Three events, unique to meiosis, occur during the first division cycle.
1. During prophase I, homologous chromosomes pair up in a process called synapsis.
A protein zipper, the synaptonemal complex, holds homologous chromosomes together tightly.
Later in prophase I, the joined homologous chromosomes are visible as a tetrad.
At X-shaped regions called chiasmata, sections of nonsister chromatids are exchanged.
Chiasmata is the physical manifestation of crossing over, a form of genetic rearrangement.
2. At metaphase I homologous pairs of chromosomes, not individual chromosomes are aligned along the metaphase plate.
In humans, you would see 23 tetrads.
3. At anaphase I, it is homologous chromosomes, not sister chromatids, that separate and are carried to opposite poles of the cell.
Sister chromatids remain attached at the centromere until anaphase II.
The processes during the second meiotic division are virtually identical to those of mitosis.
Meiosis I:
Prophase I - pairing of homologous chromosomes
Metaphase I – homologous pairs line up at equator
Anaphase I – homologous chromosomes separate
Telophase I – daughter cells are haploid
The events of meiosis II are like those of mitosis except in meiosis II, the nuclei contain the haploid number of chromosomes.
At the end of telophase II of meiosis II, there are four haploid daughter cells that are not genetically identical.
At the end of mitosis , there are two diploid daughter cells that are identical.
