2.2 All cells arise from other cells

State what the cell cycle is and outline its
stages.

cycle of division with intermediate growth
periods

1. interphase
2. mitosis or meiosis (nuclear division)
3. cytokinesis (cytoplasmic division)

Explain why the cell cycle does not occur
in some cells.

After differentiation, some types of cell in
multicellular organisms (e.g. neurons) no
longer have the ability to divide.

What is the difference between the cell
cycle and mitosis?

Cell cycle includes growth period
between divisions

mitosis is only 10% of
the cycle & refers only to nuclear
division.

Outline what happens during interphase.

G1: cell synthesises proteins for replication e.g.
tubulin for spindle fibres & cell size doubles
S: DNA replicates = chromosomes consist of 2
sister chromatids joined at a centromere
G2: organelles divide

State the purpose of mitosis.

produces 2 genetically identical daughter cells
for:
● Growth
● Cell replacement/ tissue repair
● Asexual reproduction

Name the stages of mitosis.

1. Prophase
2. Metaphase
3. Anaphase
4. Telophase

Outline what happens during prophase.

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.

Outline what happens during metaphase.

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

Outline what happens during anaphase.

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.

Outline what happens during telophase.

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.

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).

Outline how to prepare a temporary
mount of root tissue.

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 45° to press down coverslip
   & obtain a single layer of cells. Avoid trapping air
   bubbles.

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.

How do prokaryotic cells replicate?

Binary fission:

1. DNA loop replicates. Both copies stay attached to cell
   membrane. Plasmids replicate in cytoplasm.
2. Cell elongates, separating the 2 DNA loops.
3. Cell membrane contracts & septum forms.
4. Cell splits into 2 identical progeny cells, each with 1 copy
   of the DNA loop but a variable number of plasmids.

Estimate the exponential growth of
bacteria within 8 hours. Assume binary
fission occurs once every 20 minutes &
there is 1 bacterium at the start.

8 x 60 = 480 mins
480 / 20 = 24 divisions
2^24

Why are viruses classified as non-living?

They are acellular: no cytoplasm, no
metabolism & cannot self-replicate.

Outline how viruses replicate.

1. Attachment proteins attach to receptors on host cell
   membrane.
2. Enveloped viruses fuse with cell membrane or move in via
   endocytosis & release DNA/ RNA into cytoplasm OR
   viruses inject DNA/ RNA.
3. Host cell uses viral genetic information to synthesise new
   viral proteins/ nucleic acid.
4. Components of new viral particle assemble.

How do new viral particles leave the host
cell?

a) Bud off & use cell membrane to form
envelope.
b) Cause lysis of host cell.

Why is it so difficult to develop effective
treatments against viruses?

Replicate inside living cells = difficult to
kill them without killing host cells.