TERMTERM ChromosomeChromosome

A structure made of DNA and histonesA structure made of DNA and histones

DNA (deoxyribonucleic acid)DNA (deoxyribonucleic acid) A polynucleotide that contains the pentose A polynucleotide that contains the pentose

sugar deoxyribose.sugar deoxyribose.

ChromatidChromatid One of two identical parts of a chromosome One of two identical parts of a chromosome

held together by a centromereheld together by a centromere

CentromereCentromere The centralize region joining two sister The centralize region joining two sister

chromatidschromatids

GenomeGenome The complete complement of an organism’s genesThe complete complement of an organism’s genes An organism’s genetic materialAn organism’s genetic material

ChromatinChromatin Content of the nucleusContent of the nucleus Consisting of protein, DNA and RNA, which form Consisting of protein, DNA and RNA, which form

threadsthreads

EuchromatinEuchromatin Loosely packed chromatinLoosely packed chromatin

HeterochromatinHeterochromatin Densely packed chromatinDensely packed chromatin

CytokinesisCytokinesis Division of the cytoplasm during cell divisionDivision of the cytoplasm during cell division

KaryokinesisKaryokinesisDivision of the nucleus during cell divisionDivision of the nucleus during cell division

GeneGeneFunctional unit of heredityFunctional unit of heredityComposed of DNA which carries Composed of DNA which carries

information from one generation to the information from one generation to the nextnext

GameteGameteA haploid cell, formed by meiotic cell A haploid cell, formed by meiotic cell

division of a germ celldivision of a germ cellRequired for sexual reproductionRequired for sexual reproduction

The concept of cell division

• Basic characteristics of cell division

• Characteristics of cell survival, DNA and chromosome

THE CONCEPT OF CELL DIVISION.

• Modern cell theory states that ‘All new cells are derived from other cell’.

– All cells which comprise a human are derived, through the cell division, from single zygote formed by the fusion of two gametes.

– These gametes in turn were derived from the division of certain parental cell.

There are two basic types:

1.Mitosis which results in all daughter cells having the same number of chromosomes as the parent.

2. Meiosis which results in the daughter cells having only half the number of chromosomes found in the parent cell.

• Cell division – involves the distribution of identical genetic material

(DNA) to two daughter cells.

• A dividing cell – duplicates its DNA, allocates the two copies to

opposite ends of the cell– then split into daughter cells.

The cell cycle

• Description of the four stages in the cell cycle

• General explanation events during the G1, S and G2 followed by events in the mitotic phase.

THE CELL CYCLE.1. The complete sequence of

events in the life of an individual diploid cell.

2. The four stages of the cell cycle;

i. G1• The first Gap Phase

ii. S Phaseiii. G2

• The second Gap phase

iv. Mitotic Phase

i. G1 - The first growth phase

• The longest phase• Volume of cytoplasm

increase• Protein synthesis• Increase number of

organelles

ii. S phase

• DNA synthesis phase• The cell’s DNA replicates

and non consist of two identical chromatids.

iii. G2 - The second growth phase

• Energy stores are increased.

iv. Mitosis

• This process of nuclear division and followed by division of cytoplasm called cytokinesis.

Mitosis

• Explain the mitotic cell division using diagrams and photographs

• Show the position of the chromosomes at each stage

• State the changes in the chromosomes• Describe briefly the cytokinesis process and list

the differences between cell division in animal and plant cells

• Significances of mitosis

MITOSIS

2 phases : i. nucleus division ( karyokinesis )ii. cytoplasm division ( cytokinesis )

4 stages: i. prophaseii. metaphaseiii. anaphaseiv. telophase.

Mitosis in a generalized animal cell.

Prophase• Chromosomes

– visible as long, thin tangled threads.– shorten and thicken– comprise two chromatids joined at the

centromere.

• Centrioles – migrate to opposite ends of poles of the cell

(except for plant). – 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.

• The nucleolus disappears

• Nuclear envelope disintegrates

Mitosis in a generalized animal cell.

Metaphase• The chromosomes

arranged themselves at the metaphase plate, and become attached to certain spindle fibers at the centromere.

• Contraction of these fibers draws the individual chromatids slightly apart.

Mitosis in a generalized animal cell.

Anaphase• The centromeres split and further shortening of the spindle fibers causes the two

chromatids of each chromosome to separate and migrate to opposite poles.

• 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.

Mitosis in a generalized animal cell.

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.

Mitosis in a generalized animal cell.

Cytokinesis – division of cytoplasmIn Animal Cells• Occur by a process known

as cleavage.

• The first sign of cleavage is the appearance of cleavage furrow. (Begins as a shallow growth in the cell surface)

Cytokinesis – division of cytoplasmIn 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.

Mitosis in a generalized animal cell.

Differences between mitosis in plant and animal cells

Animal Cells Plant Cells

1. Involve aster or spindle formation.

1. Do not form centriols and lack centriols.

2. Cytokinesis occurs by the constriction of microtubules – cleavage furrow.

2. Occurs by the growth of a cell plate through the fusion of vesicles.

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.

Significance of mitosisGrowth• The number of cell within organism increases by mitosis and this is the

basis of growth in multicellular organisms.

Cell replacement• Replacement of cells and tissues involves mitosis.

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.

Asexual reproduction• Mitosis is the basis of asexual reproduction, the production of new

individuals of a species by one parent organism.

Questions

A

B

Figure 1

Figure 1 shows the phase in two type of cell division and

each cell contain four chromosomes

(a) i. Name the phase in cell A (1M)

ii.State 2 events which occur before the phase

that you name in a(i). (2M)

iii. State the type of cell division in cell A (1M)

iv. State 2 significant of a(iii). (2M)

(b) i. Name the phase in cell B (1M)

ii. Give the reason for your answer in b(i). (1M)

(c) Name the organ which undergoes cell division in a(iii).

In plant : (1M)

In animal :(1M)

Meiosis

• Explain the processes in Meiosis I and Meiosis II using diagrams and photographs

• Show the position and changes of the chromosomes during each stage

• Define chromatid, synapsis, bivalent, tetrad, chiasma, cross-over and centromere

• Compare and contrast meiosis and mitosis

CONTENT OF MEIOSIS Meiosis (meio, to reduce)

a form of nuclear division in which the chromosome number is halved from the diploid number (2n) to the haploid number (n).

involves DNA replication during interphase in the parent cell,

followed by two cycle of nuclear divisions and cell division,

• meiosis I

• meiosis II

Thus a single diploid cell gives rise to four haploid cells.

Meiosis occurs during the formation of

sperms and eggs (gametogenesis) in animal and during spores formation in plants.

a continuous process but is conveniently divided into

• prophase, • metaphase, • anaphase• telophase.

These stages occur in the first meiotic division and again in the second meiotic division.

Prophase I Metaphase I Anaphase IInterphase

Meiosis

Telophase I Metaphase IIProphase II Anaphase II

Telophase II

Terms • Sister chromatid

– Two identical chromatid which are held together at the centromere.• Synapsis

– Pairing • Bivalent

– A pair of homologous chromosomes jointed by proteins of the synaptonemal complex and chiasma– Each chromosome consists of two chromatids and therefore, each bivalent consist of four chromatid.

• Tetrad– a pair of homologous chromosomes with four chromatids

• Homologous chromosome– A pair of chromosome which have the same pattern of genes along the chromosome but the nature of

the genes may differ.– One member of each pair comes from female parent and the other from male.

• Chiasma– The region of cross-over at which two non-sister chromatid are joined.

• Crossing over– An exchange of portions of chromatids between homologous chromosome.

• Haploid– A single set of unpaired chromosomes

• Diploid – Two sets of chromosomes are present, one set being derived from the female parent and the other

from male.

MEIOSIS I

MEIOSIS I

4 phase– Prophase I– Metaphase I– Anaphase I– Telophase I

Prophase I

The longest phase. This phase can be divided into 5 stages;

a) Leptotene

b) Zygotene

c) Pachytene

d) Diplotene

e) Diakinesis

i. Leptotene– The beginning of first prophase of meiosis– Chromatids can be seen and pairing begins

ii. Zygotene– The second phase of the first prophase of meiosis– Pairing (synapsis) of homologous chromosomes takes places – Intimate contact is made between identical regions of

homologous, in a process involving proteins and DNA organized to form a synaptonemal complex.

iii. Pachytene– Paired homologous chromosomes are fully contracted and

twisted around each other.

iv. Diplotene– Paired homologous chromosomes begins to move apart– They remain attached at a number of points (chiasma)

v. Diakinesis– The period at the end of the first prophase of meiosis– The separation of homologous chromosomes is almost complete

and crossing over has occurred.

Prophase I

• All chromosomes are fully contracted and deeply stained;

• The centrioles (if present) have migrated to the poles.

• Chiasma and crossing over occurs.

• The nucleoli and nuclear envelope have dispersed.

• Lastly, the spindle fibers form.

Metaphase I

• The bivalents become arranged around the metaphase plate, 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.

• Spindle and spindle fibers usually disappear.

• Cleavage (animals) or cell wall formation (plants) then occurs as in mitosis.

Prophase I Metaphase I Anaphase IInterphase

Meiosis

Telophase I Metaphase IIProphase II Anaphase II

Telophase II

MEIOSIS II

MEIOSIS II

4 phase– Prophase II– Metaphase II– Anaphase II– Telophase II

Interphase II

• This stage is present usually in animal cell and varies in length.

• No further DNA replication occurs.

Meiosis II is similar to mitosis.

Prophase II

This stage is absent if interphase II 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.

They are arranged at right – angles to the spindle of meiosis I.

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.

Sister chromatids separate

Telophase II

• As telophase in mitosis but 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).

• Subsequent cleavage (animals) or cell wall formation (plants) will produce four daughter cells from the original single parent cell.

SIGNIFICANCE OF MEIOSIS

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.

Chiasma 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. During metaphase I – homologous pairs of chromosomes, not individual

chromosomes are aligned along the metaphase plate.

– In humans, you would see 23 tetrads.

3. During 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.

Ǿ The processes during the second meiotic division are virtually identical to those of mitosis.

COMPARISON BETWEEN MEIOSIS AND MITOSIS

MITOSIS MEIOSIS

1. Occurs in soma cell. Occurs in gonad cells (ovaries in females and testes in males)

2. Conserves chromosome number (2n) replicated chromosomes.

Reduces the chromosome number by half (n) non-replicated chromosomes.

*gonad : any of usually paired organs in animals that produced reproductive cell (gametes).

COMPARISON BETWEEN MEIOSIS AND MITOSIS

MITOSIS MEIOSIS

3. By the end of prophase, no synapsis occur to form bivalent.

Synapsis occurs to form bivalent at the homologous chromosomes during prophase I

4. No chiasma occurs so there is no cross – over.

Some chiasma occurs to form cross – over.

Genetic variability is a result from the cross – over.

COMPARISON BETWEEN MEIOSIS AND MITOSIS

MITOSIS MEIOSIS

5. The contain of genetic in daughter cell is identical in parental cells.

The contain of genetic in daughter cell is no identical as the parental cell.

6. Two daughter cells each diploid (2n)

Four daughter cells each haploid (n)

7. Cytokinesis occurs once.

Cytokinesis occurs once or twice.

8. The daughter cell can produces mitosis.

The daughter cell can produces mitosis but not meiosis.

MITOSIS PRODUCES 2 IDENTICAL DAUGHTER CELLS BUT

MEIOSIS PRODUCES 4 NON-IDENTICAL DAUGHTER CELLS

THAT’S ALL FOR THIS TOPIC