3- Eukaryotic cell cycle and division

What is the cell cycle?

A regulated process in which cells divide to produce two genetically identical daughter cells for growth, repair and asexual reproduction.

As all the cells produced by mitosis are genetically identical, mitosis does not give rise to genetic variation.

What are the three stages of the cell cycle?

1. Interphase

2. Mitosis or meiosis

3. Cytokinesis

What are the stages of mitosis?

1. Prophase

2. Metaphase

3. Anaphase

4. Telophase

(Please, Make, A, Taco)

What happens during interphase?

G1- cell synthesises proteins for replication e.g. tubulin for spindle fibres and cell size doubles.

S- DNA replicates= chromosomes consist of 2 sister chromatids joined at a centromere.

G2- organelles divide.

What is the purpose of mitosis?

To produce 2 genetically identical daughter cells for:

• growth

• cell replacement/ tissue repair

• asexual reproduction

What happens during prophase (in mitosis)?

1. Chromosomes condense, becoming visible. (X-shaped: 2 sister chromatids joined at centromere).

2. Centrioles move to opposite poles of cell (animal cells) & mitotic spindle fibres form.

3. Nuclear envelope & nucleolus break down = chromosomes free in cytoplasm.

What happens during metaphase (in mitosis)?

Sister chromatids line up at cell equator, attached to the mitotic spindle by their centromeres.

What happens during anaphase (in mitosis)?

Requires energy from ATP hydrolysis

1. Spindle fibres contract= centromeres divide.

2. Sister chromatids separate into 2 distinct chromosomes & are pulled to opposite poles of cell. (looks like ‘V’ shapes facing each other).

3. Spindle fibres break down.

What happens during telophase (in mitosis)?

1. Chromosomes decondense, becoming invisible again.

2. New nuclear envelopes form around each set of chromosomes = 2 new nuclei, each with 1 copy of each chromosome.

What happens during cytokinesis (in mitosis)?

1. Cell membrane cleavage furrow forms.

2. Contractile division of cytoplasm.

Explain how to prepare a temporary root tip squash.

1. Place root in hydrochloric acid to halt cell division & hydrolyse middle lamella.

2. Stain root tip with a dye that binds to chromosomes.

3. Macerate tissue in water using mounted needle.

4. Use mounted needle at 45degrees to press down coverslip & obtain a single layer of cells. Avoid trapping air bubbles.

Explain the procedure for a root tip squash experiment.

1. Prepare a temporary mount of root tissue.

2. Focus an optical microscope on the slide. Count

total number of cells in the field of view and number

of cells in a stage of mitosis.

3. Calculate mitotic index (proportion of cells

undergoing mitosis).

Name 2 dyes that bind to chromosomes.

toluidine blue (blue)

acetic orcein (purple-red)

Why is only the root tip used when calculating a

mitotic index?

Meristematic cells at root tip are

actively undergoing mitosis.

Cells further from root tip are elongating

rather than dividing.

What is meiosis?

A form of cell division that produces four genetically different haploid cells (cells with half the number of chromosomes found in the parent cell) known as gametes.

What happens during meiosis I ?

1. Homologous chromosomes pair to form bivalents.

2. Crossing over (exchange of sections of genetic

material) occurs at chiasmata.

3. Cell divides into two. Homologous chromosomes

separate randomly. Each cell contains either

maternal or paternal copy.

What happens during meiosis II?

1. Independent segregation of sister

chromatids.

2. Each cell divides again, producing 4

haploid cells.

How does meiosis produce genetic variation?

• Crossing over during meiosis I .

• Independent assortment (random segregation) of homologous chromosomes & sister chromatids.

Result in new combinations of alleles.

What is a chromosome mutation?

Involves section of a chromosome rather than 1 base.

Types include: deletion, inversion, duplication, translocation.

What is a translocation mutation?

Where a part of one chromosome breaks off and reattaches to another completely different chromosome. Can be balanced or unbalanced.

What is a non-disjunction mutation?

Chromosomes fail to separate correctly in meiosis.

Gametes have at least one more (polysomy) or one less (monosomy) chromosomes than normal.

What is a deletion mutation?

A section of a chromosome is removed, resulting in the loss of a large number of genes.

What is a inversion mutation?

A part of a chromosome flips its orientation with respect to the rest of the chromosome.

Give examples of conditions caused by

non-disjunction.

Trisomy-21 = Down's syndrome.

X-monosomy of sex chromosomes =

Turner's syndrome.