D2.1 Cytoplasm and Nuclear division

Cytokinesis in animal cells

The final stage of cell division when the membrane folds inwards and a ring of actin and myosin contracts and pinches membrane together

• This is the division of the cytoplasm

• Forms a cleavage furrow - occurs when an animal cell membrane folds inwards

• Actin and myosin for a contractile ring → the membrane pinches off to form 2 cells

• Organelles are distributed between both ends of the cell so organelles in both cells are formed

• Mitochondria (and chloroplasts in plant cells) must move to each side as the new cell cannot synthesise these

Cytokinesis in plant cells

• Vesicles fuse together to form a plasma membrane and cell wall

• Golgi vesicles containing carbohydrates collect along the centre of the cell to form a cell plate

• Plant cells cannot form a cleavage furrow due to the presence of a cell wall

Oogenesis - unequal division of cytoplasm

• This is the formation of eggs in ovaries

• Primary oocytes divide to form secondary oocytes + a smaller polar body

• This occurs before fertilisation

• Secondary oocytes divides again to form an ovum and a second polar body

• The final large ovum absorbs the cytoplasm + contents of polar bodies to form one large egg

Budding

• A method of sexual reproduction in some species

• Swelling forms out the side (a bud)

• The bud continues to grow and splits off

• Nucleus copies by mitosis - one nucleus moves into the bud

• Scar is left behind on the parent cell

Mitosis + meiosis similarities

• Both nuclear division

• Nucleus must divide before cytokinesis or the cell will be anucleate

• DNA replication is therefore a prerequisite for both

Products + uses of mitosis

• 2 genetically identical cells

• Forms diploid cells

• 1 division

• Used for growth + to replace cells and for asexual reproduction

Products + uses of meiosis

• 4 genetically different cells

• Forms haploid cells

• 2 divisions

• Used to make gametes

Condensation of chromosomes

• DNA is coiled around histone proteins

• 8 histones + DNA = a nucleosome

• Histone = positively charged so attracts negatively charged DNA

• Supercoiling occurs - DNA further twists around nucleosomes to form tight coiled structure

Movement via microtubules

• Microtubules are a part of the cytoskeleton

• They can lengthen and shorten via polymerisation and depolymerisation

• Microtubule motors = also needed to move chromosomes to cell centre

Mitosis: Prophase

• Nuclear envelope + nucleolus breaks down

• Genetic material now in cytoplasm

• Chromosomes condense - supercoiling of DNA

• Centrioles form at opposite poles

Mitosis: Metaphase

• Chromosomes are bound to spindle fibres at the centriole

• Chromosomes line up at the equator of the cell

Mitosis: Anaphase

Chromatids are pulled by the spindle fibre to opposite poles

Mitosis: Telophase

• Chromatids decondense

• 2 nuclear membranes reform - 2 nuclei

• spindle fibre breaks down

Mitotic index

• Used to calculate how regularly cells divide

• Can determine whether cells are cancerous

• Mitotic index = cells in mitosis/total number of cells

Why mitosis is important

It is nuclear division, important for human life and needed for:

• Growth/production of extra body cells

• Development of an embryo

• Wound healing/tissue repair/cell replacement

• Clonal selection/division of lymphocytes for antibody production during immune response

• First stage of gametogenesis (forming sperm and egg)

Meiosis: Prophase 1

• Nuclear membrane breaks down

• Chromosomes condense

• Chromosomes form ‘homologous pairs’ called bivalents

• Crossing over may occur

Meiosis: Metaphase 1

• Homologous pairs line up at equator

• Spindle fibres connect

Meiosis: Anaphase 1

• Spindle fibres contract + shorten

• Homologous chromosomes are pulled to opposite poles

Meiosis: Telophase 1

• Chromosomes decondense

• Nuclear membrane reforms

• 2 haploid cells are formed

Meiosis: Prophase 2

• Nucleolus and nuclear membrane breaks down

• Chromosomes condense

• Spindle fibres form perpendicular to prophase 1 and attach to a single chromosome

Meiosis: Metaphase 2

Chromosomes line up at the equator of thecell

Meiosis: Anaphase 2

• Chromatids are moved to opposite poles

• Occurs due to spindle fibred condensing and shortening

Meiosis: Telophase 2

• Chromosomes decondense

• Nuclear membrane reforms

• 4, genetically different haploid cells are produced

Haploid definition

A cell containing a single set of chromosomes

Crossing over (recombination)

• A cause of variation during meiosis

• Occurs during prophase 1

• When homologous chromosomes are paired up, non-sister chromatids come into contact with each other

• Contact point is called the chiasma

• An enzyme cuts and swaps the chromosomes

• This creates a new combination of alleles and forms recombinant chromatids

• Recombinant chromatids = chromatid that has information crossed over, a combination of both chromatids DNA

Independent assortment (random orientation of bivalents)

• Causes variation during meiosis

• The arrangement of chromosome combinations during metaphase 1 lead to different chromosome combinations

• This means different combinations of chromosomes are pulled to opposite ends of the cell

• Can also occur for chromatids during metaphase 2, although only relevant if crossing over has occurred

Random fusion of gametes

• Causes variation

• Which sperm fertilised the egg is random, creating variation

Non-disjunction

• Occurs when there is a mistake in meiosis

• Happens if chromosomes in anaphase 1 or chromatids in anaphase 2 fail to separate

• This can cause gametes to have an incorrect number of chromosomes

• Down syndrome occurs when there is an extra chromosome - 2 copies of chromosome 21

Factors affecting non-disjunctions

• Chances of non-disjunction increases as parental age increases

• Particularly strong correlation between maternal age and occurrence of non-disjunction

• Risk of chromosomal abnormalities in offspring increase significantly after the age of 30

• There is a higher risk of of chromosomal errors in offspring as a result of non-disjunction in meiosis 1

Cell proliferation and meristems

• Cell proliferation is the increase in the number of cells, caused by cell growth and cell division

• Meristem cells - retain the ability to divide by mitosis to form new cells/tissues

• Apical meristem cells - cause lengthening and upwards growth, occur at tips of shoots and roots

• Lateral meristem cells - located at the cambium, causes the stem to grow wider

Cell proliferation in embryos

• Number of cells in the embryo doubles with each division

Cell proliferation - replacement in wound healing

• Dermal cells divide - make more cells

• new cells migrate upwards towards the skin surface

• Dead cells on the surface are replaced and fall off

Cell replacement - wound healing

• Clotting occurs - fibrin matrix + platelets form a clot

• During proliferation, stem cells are activated in the dermal layer. These regenerate new cells to repair the wound

The phases of interphase

Consists of gap phases (G1 and G2) and a synthesis phase (S) in between

Interphase: G1

• Cellular contents (excluding chromosomes) are duplicated

• Protein synthesis occurs

• Cell doubles in size

• Cytoplasm volume is increased

Interphase: S

All 46 chromosomes are duplicated

Interphase: G2

• The cell proofreads the duplicated chromosomes, making necessary repairs

• Microtubules are synthesized

What the G1 checkpoint checks for

• Cell size

• DNA damage

• Growth factors

• Nutrients

What the G2 checkpoint checks for

• Cell size

• DNA replication

What the spindle assembly checkpoint checks for

Chromosome attachment to the spindle

The importance of checkpoints during the cell cycle

• Important to ensure the cell only divides when it has grown to the right size, DNA is error free and chromosomes in the correct positions

• To ensure this, checkpoints in the cell cycle act as control mechanisms

• The cell cannot proceed to next cell cycle stage unless they have completed the previous one

The roll of cyclins

• Cyclins are chemicals that control the cell cycle

• There are 4: D, E, A and B

• Their levels rise and fall throughout the cell cycle - determines what stage the cell cycle is in

Each cyclin and their roll

D - Triggers the cell to move from G0 to G1 and from G1 to S phase

E - Prepares the cell for DNA replication in S phase

A - activates DNA replication inside the nucleus in S phase

B - Promotes the assembly of the mitotic spindle and other tasks in the cytoplasm to prepare for mitosis

Cyclins and kinase enzymes

• Each cyclin activates a different cyclin dependent kinase enzyme (CDK)

• The CDK then activates a target protein, regulating the cell cycle

• The cyclins are then destroyed

Mitosis promoting factors

• A complex between cyclin B and an associated CDK

• This triggers condensation of DNA, spindle formation, nuclear membrane breakdown e.t.c

Serendipity

• A happy and unexpected discovery made by accident

• Discovery of cyclins by Tim Hunt is an example of serendipity in science

• He was awarded the physiology nobel prize in 2001 for this

Cancer

• Uncontrolled cell division that causes tumour formation

• May occur if cells divide at a faster rate than are destroyed

Cancerous vs normal cells

Cancer:

• Large, variable shaped nuclei

• small cytoplasmic volume

• Variation in cell size and shape

• Disorganised arrangement

• Larger levels of dividing cells

Normal:

• small, uniform nuclei

• Large cytoplasmic volume

• Conformity in cell size and tissue

• Cells arranged in discrete tissues

• Lower levels of dividing cells

Primary and secondary cancer

• A mutation causes rapid cell division

• Results in a group of rapidly dividing cells called a primary tumour

• Vascularisation occurs - blood vessels divert towards the tumour and surround it

• One cancerous cell may detach, enter blood and go to a different organ

• This causes the formation of a new, secondary tumour elsewhere in the body

Benign vs malignant tumours

Benign tumours = non cancerous, cannot spread to other parts of the body

Malignant tumour = cancerous tumour that may spread to other sides via the blood stream and lymphatic system

Metastasis

• Can occur in primary tumours

• Is the process when cells break away and secondary tumours form in other areas

Cancer and oncogenes

• Proto-oncogene and tumour repressor gene control cell division

• Proto-oncogene stimulates cell division, tumour suppressor gene slows it down

• If these occur at the same rate, normal cell division occurs

• If a proto-oncogene mutates to form an oncogene - there will be uncontrolled cell division and cancer

• If the tumour suppressor gene is inactivated, there will be uncontrolled cell division and cancer

Mutagens

• An agent, such as radiation or a chemical, that causes a mutation

• The chance of one mutation causing cancer is small

• As there are many cells in the body, the total chance of tumour formation is higher over a lifetime

Cigarettes → cancer

• There is a strong correlation between cigarettes and cancer

• Cancer is a global problem