cell division and cell cycle

Question 1

Bacterial Division

Answer 1

1. Bacteria divide by binary fission.(although exchange DNA, they dont have sexual cyclic like eukaryotes)
2. chromosomes are tightly packed
   - packaging via structural maintanence protein (SMC)
   - SMC has a role in DNA organisation

Question 2

Eukaryotic chromosomes

Answer 2

1. They divide by the process of mitosis
2. The chromosome set in a single organism is calle “KARYOTYPE”
3. Human has 46 chromosomes
   - Organised in a set of 2 copies of 23 Chromosomes
4. One set is called a “HAPLOID” number
5. Two copies makes “DIPLOID”
   - Two matchig Chromosomes are homologus

Question 3

Eukaryotic chromosomes

Answer 3

1. They have a complex multi-layered structure
2. Chromosomes are composed of chromatin
   - A Chromatin is composed of DNA and Proteins (40% DNA & 60% Proteins)
   - RNA must also be present for transcription
3. In nondividing structures can be :
   - Heterochromatin meaning that the region of chromatin is inactive
   - Euchromatin meaning that the region of chromatin is active
   - These relates the actual sructure of a chromosome

Question 4

Chromosome Structure

Answer 4

1. DNA double helix/duplex (Naked DNA)
2. Beads on a string
3. Nucleosome (with a histone core & DNA)

Question 5

Nucleosome

Answer 5

1. The beads in the Beads-on-string
   - Are composed of DNA and Histone proteins
   - Histone proteins are H2A, H2B, H3 & H4
   - Form complex of 8 proteins
2. Histones are positively charged
   - attracts negatively charged DNA
3. 147bp wrapped in 1.7 turns around Histone
   - Forms nucleosome with 10nm diameter
4. nucleosome linked by linker region
   - 20-80bp in length

Question 6

Beads-on-string

Answer 6

Notice the 8 Histone cores and their role in Nucleosome formation

Question 7

Chromosome structure (2.0)

Answer 7

1. Nucleosome wrapped into high order coils with 30nm diameter
   - They are modeled as solenoids
   - Assumed state of nondividing chromatin
2. Chromosomes in solenoid are more tightly packed
   - Chromatin arranged around scafold of protein to achieve maximum compaction
3. Exact model of DNA compacting unclear
   - Evidence for model lacking
4. Condensin proteins are crucial
   - Uses energy (ATP) to control DNA looping
5. DNA packaging relies on Nucleosomes and Histones to change the density

Question 8

Chromosome Replication

Answer 8

1. Each chromosome usually consists of a single double stranded DNA molecule
2. After replication, each chromosome composed of 2 identical DNA molecules
   - Held together by Cohesin protein at the Centromere
3. Further packaging produces visible chromosome structure
   - Now one chromosome is composed of two sister chromatids
4. Cohesin ensures that sister chromatids remains attached until planned separation
   - Each gets equal genetic material

Question 9

The cell cycle

Answer 9

1. G1, S & G2 (Interphase)

2. Mitosis, Cytokinesis (M phase)

Question 10

G1 PHASE

Answer 10

Primary growth phase, longest phase

Question 11

S (SYNTHESIS)

Answer 11

Replication of DNA

Question 12

G2 PHASE

Answer 12

Prepare for DNA division, microtubule organise

Question 13

MITOSIS

Answer 13

1. Subdivided into 5 phases
   - prophase
   - prometaphase
   - metaphase
   - anaphase
   - telophase
2. Important in chromosome segragation

Question 14

CYTOKINESIS

Answer 14

Separation of 2 new cells

Question 15

G0 Phase

Answer 15

1. Resting phase

2. Most ceels are in G0 e.g. Muscle and neural cells

Question 16

INTERPHASE

Answer 16

1. G1 & G2 phases of active growth, protein synthesis and cell organelle duplication.
2. S phase for chromosome replication.
   - Chromatids joined by cohesin.
3. Chromosomes condense in G2
   - In G2 makes machinery to move chromosomes.
   - Animals cell make two centrioles as microtubule organizing centre.
   - All eukaryotes make tubulin for microtubules.

Question 17

PROPHASE

Answer 17

1. Starts when individual chromosomes are visible
   - condensation continues throughout.
2. Spindle apparatus forms
   - Replaces all microtubules
   - Spindle microtubules will move chromosomes around
3. Centrioles are important for spindle apparatus in animal cell
   - Microtubules between centrioles and cell membrane are called ASTERS.
   - Braces microtubules against the cell membrane.
4. No Asters in plants
   - Rigid cell wall
5. Nuclear membrane breaks down.
   - Recycled by ER.

Question 18

PROMETAPHASE

Answer 18

1. Characterised by a dissolved nuclear membrane.
2. Kinetochores attach to microtubules
   - Aligns the chromosomes at the equator.
   - Important for a later split of chromatids
3. Chromosome movement by assembly and disassembly of microtubules.
4. Motor proteins at kinetochores.

Question 19

METAPHASE

Answer 19

1. characterized by the alignment of chromosomes at the equator.
   - Along the inner circumference of the cell.
   - Imaginary plate through this is called a Metaphase plate
2. Indicates the plane of cell division.
3. Aligned chromosomes have centromere between two ends
   - Ready for chromatid separation.

Question 20

ANAPHASE

Answer 20

1. Begins when the cohesion proteins at the centromeres are removed
2. Sister chromatids are now pulled to opposite poles
3. Movement occurs in 2 phases: Anaphase A
   - Kinetochores are pulled towards poles
   - Driven by microtubule shortening through subunit removal
4. Anaphase B
   - Poles move apart
   - Cell physically elongates

Question 21

TELOPHASE

Answer 21

1. Spindle apparatus disassemble
   - Tublin recycled to form the cytoskeleton
2. Nuclear envelope forms around each set of sister chromatids
   - Now called chromosomes
3. Nucleolus reappears in each nucleus
4. Cell organelles move toward poles