Cytogenetics - Mitosis and Meiosis

Mitosis

• process by which the somatic cells of all multicellular organisms multiply

• occurs only in eukaryotes.

Somatic cells

non-reproductive cells of which an organism is composed.

gametes

are sexual reproductive cells, that is, there are two types, male and female.

mature organism

In sexual reproduction, a male gamete combines with a female gamete and the resulting, merged cell then divides repeatedly by mitosis to eventually produce a —

mitosis

Plants produce gametes by —

spores

Plants also make asexual reproductive cells through meiosis called —

mitosis

A single spore, produced by meiosis, develops into a mature organism by. —

growth, cell replacement, regeneration, vegetative reproduction/asexual reproduction

Why mitosis? (significance of mitosis)

growth

The number of cells within an organism increases by mitosis and this is the basis of growth in multicellular organisms.

Cell replacement

Cells are constantly sloughed off, dying and being replaced by new ones in the skin and digestive tract.

Regeneration

Some animals can regenerate parts of the body, and production of new cells are achieved by mitosis

Vegetative Reproduction

Some plants produce offspring which are genetically similar to themselves.

clones

genetically identical offisprings

• No variation in genetic information

• No variation in chromosome number due to the semi-conservative replication of DNA and equal distribution of DNA.

• The cell divides once.

• two identical daughter cells are formed

Consequence of mitotic division

sister chromatids

• The two chromatids that make up a chromosome are called —

• They therefore contain identical alleles at all loci.

homologous chromosomes

• chromosomes that have the same set of loci in the same order

• usually do not have identical alleles at all loci.

• They are inherited from different parents and are not direct copies of each other

meiosis II

when are the two sister chromatids of each chromosome are segregated into separate cells

unreplicated chromosomes

As soon as the joined chromatids are separated they are no longer called sisters because they are no longer connected to each other. - They are now called —-

prophase, metaphase, anaphase, telophase

what are the stages of mitosis

Interphase

• There is intense cellular activity.

• Cell grows (growth phase)

• Chromosomes are uncondensed because they are still enclosed in the nuclear membrane.

• Period when the cell is not undergoing mitotic division

G1 phase, S phase, and G2 phase

what are the three stages of interphase?

G1 phase

chromosomes haven’t replicated yet

S phase

chromosomes replicate

G2 Phase

replication is complete

PROPHASE

• Chromosomes become visible with a light microscope as they condense.

• Spindle (yellow strands) begins to extend outward from each of two centers of extension.

• The nuclear membrane (white) begins to break up and disappear.

• Each chromosome has been duplicated and so is composed of two sister chromatids containing identical genetic information.

Aster

starlike configuration

Metaphase

Chromosomes line up in the middle of the cell.

Metaphase plate

imaginary plane halfway between the centrioles

ANAPHASE

The two chromatids of each chromosome are pulled apart by the spindle and dragged toward opposite poles of the cell.

sister

to be called a chromatid, a chromatid must be attached to its —

unreplicated chromosomes

When the chromosomes divide at the beginning of anaphase, the sister chromatids are no longer sisters because they are no longer connected. - Once they are separated, they are called —

telophase

• The chromosomes have reached the poles and the nuclear membrane begins to appear.

• A cleavage furrow appears

• By the end of this stage of mitosis, the cell has divided in two along the plane defined by the furrow.

cell plate

In most plants, instead of a cleavage furrow, a "—" forms, dividing the cell into two daughter cells.

CYTOKINESIS

• Is not one of the phases of mitosis.

• It is the division of the cytoplasm, as opposed to karyokinesis, which is division of the nucleus.

Meiosis

• Occurs in eukaryotes.

• Takes place at some point in the life cycle of the typical sexual organism

it compensates for the doubling of the chromosome number caused by fertilization.

In meiosis, what is the significance of reducing the chromosomes in half?

production of gametes; spores are produced

• In animals, meiosis occurs during the —

• In plants, it takes place when —

PROPHASE I

(Meiosis I)

• The chromosomes become visible as they shorten, coil, and thicken.

• The spindle begins to extend outward from two centers of expansion.

• In animal cells a pair of centrioles can be seen in each of these centers.

• Each chromosome is composed of two sister chromatids containing identical genetic information.

• Most complicated phase of meiosis. - It is much longer in meiosis than i

homologous chromosomes or simply homologs.

The two members of each chromosome pair are called —

tetrad

Homologs synapse to form a —

Leptotene, Zygotene, Pachytene, Diplotene, Diakinesis

Prophase I is divided into the five substages:

Leptotene

• The bouquet stage because all the telomeres tend to contact the nuclear membrane in one spot so that the looped chromosomes balloon out from that point like flower petals.

• The chromosomes are not yet fully condensed.

• Each is a thin thread of DNA (lepto- is Greek for thin and -tene is Greek for ribbon or band) along which clearly defined beads of local coiling (chromomeres) can be seen.

• During this stage both ends (telomeres) of each chromosome are turned toward, and probably attached to, the same region of the nuclear membrane.

chromatonemata

The chromosomes, while they have this threadlike form

Zygotene

homologous chromosomes begin to unite (synapse) by coming into approximate alignment

synapsis

the process of fusion that occurs between homologous chromosomes, begins at various points along the chromosomes and extends outward, zipper-fashion, until complete

Synaptonemal complex

the interface where two homologous chromosomes

Pachytene

• The two sister chromatids of each chromosome separate from each other

• Makes the chromosome look thicker

• Homologs are still paired at this point

• Crossing-over happens

Diplotene

• These initially fused non-sister chromatids begin to separate from each other.

• At each cross-over site, the two strands form an x-shaped structure called a chiasma

• Terminalization happens

Terminalization

The chiasmata begin moving toward the ends of the chromatids.

dictyotene

- In oocytes, a special, extremely prolonged form of diplotene occurs

Diakinesis

The nucleolus disappears, terminalization reaches completion, and the chromosomes coil tightly, and so become shorter and thicker.

Metaphase I

• The tetrads align on the "metaphase plate," halfway between the poles of the cell.

• Both spindle fiber attachment points (kinetochores) of each sister chromatid pair are turned toward the same pole.

ANAPHASE I

• The cell lengthens as it begins the process of division. •

• Homologs of each chromosome pair move toward opposite poles, drawn by the microtubules of the spindle apparatus

TELOPHASE I

• The chromosomes reach the poles.

• At each pole, now, there is a complete haploid set of chromosomes.

• A cleavage furrow appears.

• By the end of this stage the cell has divided in two along the plane defined by the furrow.

Interkinesis

the nuclear membrane reappears between telophase I and prophase II, and there is a period of rest.

• Each chromosome is still composed of two chromatids.

Prophase II

• The chromosomes are condensed and not yet attached to the spindle apparatus.

• If there was an interkinesis, then the nuclear membranes begin to break down again during this stage.

• The centrioles have replicated and are moving toward the poles.

METAPHASE II

• The chromosomes move to the equator ("metaphase plate") of each of the two daughter cells produced by the first meiotic division.

ANAPHASE II

The sister chromatids of each chromosome separate and move toward opposite poles.

TELOPHASE II

• The sister chromatids reach opposite poles, cytokinesis occurs

• The two cells divide to form four haploid daughter cells

• Nuclear membranes reform.

• Production and fusion of haploid gametes

• The creation of genetic variety by the random distribution of chromosomes during metaphase

• The creation of genetic variety by crossing over between homologous chromosomes

Significance of Meiosis

metaphase I

what phase do random distribution of chromosomes/ independent assortment happen?

Prophase I

what phase does crossing over between homologous chromosome happen?

Walther Flemming

who discovered mitosis

Oscar Hertwig

who discovered meiosis