5 - The Mitotic Cell Cycle 👩‍👩‍👧‍👦

Describe the changes that occur during prophase in an animal cell. (4)

1) condensation of chromatin

2) chromosomes visible as two sister chromatids joined by a centromere

3) spindle formation / microtubules assemble

4) centrioles move to opposite poles

5) disappearance of nucleolus

6) breakdown of nuclear envelope

Chromatid

one half of a duplicated chromosome

Chromosomes

threadlike structures made of nucleic acids and proteins that contain genes

histone proteins

Proteins which package dna one nucleosomes

Chromatin

Combination of DNA and proteins

Each chromatid contains how many DNA molecules?

1

Size of DNA molecule

2nm

Why mitosis is carried out instead of meiosis

1) maintains genetic stability by producing genetically identical cells

2) ensures cells retain function

3) maintains chromosome number

Nucleosome

- 11 nm wide by 6 nm long

- made of 8 histone molecules

- DNA is wrapped outside the base cylinder making 1 2/3 turns before linking to next histone

Function of telomeres

1) permit continued replication

2) prevent loss of genes

Telomeres

Repeated DNA sequences at the ends of eukaryotic chromosomes and are needed for successful division

How telomeres permit continued replication

1) copying enzyme telomerase stops a little short of the end of DNA

2) repeated bases are added which don't have useful info but allow telomerase to complete copying meaningful DNA

Mitosis

Nuclear division that produces two genetically identical daughter cells

Interphase consists of

G1, S, and G2 phases

G1 phase

cells increase in size and synthesize new proteins and organelles

S phase

- synthesis of dna

- dna in the nucleus replicated so that each chromosome consists of two identical chromatids

G2 phase

- cell continues to grow

- the new dna is checked and any errors are corrected

- preparations are made to start cell division eg sharp increase in the production of Tubulin is observed which is needed to make microtubules for mitotic spindle

Interphase

Cell grows, performs its normal functions, and prepares for division

M phase (mitosis)

Production of genetically identical cells

- growth

- cell replacement

- repair of tissues

- asexual reproduction

- immune response (cloning of B and T lymphocytes)

Prophase

- chromatin starts to condense

- nuclear envelope and nucleolus disappears

- centrosomes migrate to opposite poles of the cell and form piles of mitotic spindle

- asters and spindle fibres form

Metaphase

- centrosomes are at cell poles and organise production of spindle fibres

- chromosomes line up across equator of spindle

Anaphase

Chromatids are pulled apart by the contraction of spindle fibres and move to opposite poles of the cell

Telophase

- nuclear envelope and nucleolus reforms

- nucleus divides

Cytokinesis

Division of the cytoplasm during cell division

cytokinesis in plant cells

1) cell plate forms across equator of the cell / cellulose cell wall laid down

2) cytoplasm is divided into two

Cytokinesis in animal cells

1) contractile ring forms pushing equator of cell inwards forming cleavage furrow

2) it deepens as actin filaments in ring contract and eventually splits into two membranes

Differences in plant and animal cells mitosis

1) plant cells don't have any visible centriole or aster

2) daughter cells get separated by cell plate in plants

Centromere

- Site of attachment of spindle microtubules

- Each metaphase chromosome has two kinetochores at its centromere

kinetochore

A structure of proteins attached to the centromere that links each sister chromatid to the mitotic spindle.

Centrosomes

- Present at the poles of the spindle

- An organelle found in animal cells that acts as an MTOC for construction of the spindle

Stem cells

Cells that can divide an unlimited number of times via mitosis

Cellular changes that occur in development of cancer

1) mutation occurs in gene that controls cell division leading to formation of oncogene from protooncogene

2) this causes uncontrolled division

3) coordination of cell cycle is lost (cell does not respond to or receive signals that tell it to stop dividing)

4) loss of function and lack of specialisation occurs

5) a tumour is formed, an irregular mass of cells showing an abnormal change in shape

Roles of centrioles during mitosis

1) act as MTOCs

2) assemble / form spindle fibres during prophase

3) contraction of spindle fibres during anaphase separates sister chromatids

State the substances used to synthesise DNA during the S phase. [3]

1. ATP

2. activated / free nucleotides

3. DNA polymerase

4. DNA ligase

Describe the behaviour of the nuclear envelope during mitosis. [2]

breakdowns at prophase into vesicles, vesicles fuse to form new membranes around both sets of daughter chromosomes in telophase