Mitosis Cytokinesis First phase of growth, the cell grows and develops, and additional organelles...

Mitosis

Cytokinesis

First phase of growth, the cell grows and develops, and additional organelles are synthesised, the nucleus directs protein synthesis.

Synthesis of DNA, the DNA of the chromosomes is replicated

Second phase of growth, further synthesis of organelles occurs, chromosomes start condensing

Nuclear division

Cytoplasmic division

InterphaseCentriole

Chromosome

Nucleolus

Cell synthesizes more cytoplasm and organelles, chromosomes replicate, cell builds up large store of energy

is long and thin, cannot be seen under the light microscope

MITOSIS

ProphaseChromosome

Shortens and condenses, becomes visible under the light microscope

Nucleolus Gradually disappears

Prophase

Chromatid

Centromere

Centrioles move towards opposite poles of the cell

Prophase

Sister chromatids

Nuclear membraneBegins to break down

CentrioleMicrotubules extend from here towards the equator of the cell to form the spindle

MetaphaseSpindle fibre

Spindle equator

Spindle pole

Centromere becomes attached to a spindle fibre and moves to the equatorial plane

Chromosomes line up at the equator of the spindle

Anaphase

Centromere divides and the daughter chromosmes move to opposite poles with centromere leading

Daughter chromosome

Telophase

Nuclear membraneSurrounds daughter chromosomes

Nucleolusreforms

Chromosomes uncoil and gradually become invisible under light microscope

Division of the cytoplasm

Animal cell

Cell membrane invaginates at the mid point of the cell dividing the cell into two, cells are diploid (2N) and are identical to each other

Plant cellTiny vesicles containing cell wall materials coalesce to form the cell plate

Cell plate extends outward to the cell wall and separates cell into two

MEIOSIS

Meiosis I

Prophase I

Bivalent

Pairing of homologous chromosomes

maternal and paternal chromosomes come together and lie close to each other

Prophase I

Sister chromatids separate and become visible, they are held together by the centromere

Prophase I

Crossing over occurs between chromatids of homologous chromosomes

Chiasma

Metaphase ICentromere becomes attached to individual spindle fibre

Chromosomes lie at the equator of the spindle with members of each homologous pair facing opposite poles

Anaphase I

Members of each homologous pair move towards opposite poles

Telophase INuclear membrane reforms around the chromosomes

Cell divides into two

Prophase II

Nuclear membrane disappearing

Metaphase II

Chromosomes attached to spindle fibres by their centromere, line up at the equator of the spindle

Anaphase II

Centromeres divide and separate, pulling the sister chromatids to the opposite poles

Telophase II

Nuclear membrane forms around the chromosomes

Interphase

Four haploid (N) daughter cells are produced

Comparison between mitosis and meiosis

Mitosis MeiosisPairing of

homologous chromosomes

No Yes

Crossing over No Yes

Anaphase Sister chromatids separate during anaphase

Homologous chromosomes separate during anaphase I, sister chromatids separate during anaphase II. Number of

divisions involved One Two

Daughter cells Two diploid (2N) daughter cells, they are identical

Four haploid (N) daughter cells, they are non-identicalOccurrence During growth,

asexual reproduction

Production of gametes for sexual reproduction

• Clegg C.J., Mackean D.G., Advanced Biology: principles and applications

• Enger/Ross, Concepts in Biology

CROSSING OVER - exchange of genetic material between non-sister chromatids

Non-sister chromatids overlap

Once overlap, they break

Join with broken fragments of non-sister chromatid i.e. exchange of genetic material occurs

Crossing over - ChiasmaChiasmata have two functions: (1) hold the homologous chromosomes together as they move to the equator of the spindle (2) allow exchange of genetic materials between homologous chromosomes giving rise to genetic variations in the offspring