Lecture 5: Cytogenetics I; Chromosome, mitosis and the Eukaryotic Cell Cycle

Question 1

How is DNA packed into chromosomes?

Chromatin, Histones, Nucleosomes, Chromatosome, Lengths, Loops, folds/compression, Chromatid, Chromosome.

Answer 1

1. At the simplest level: CHROMATIN is a DOUBLE Stranded Helical structure of DNA.
2. DNA is complexed with histones to form nucleosomes.
3. Each NUCLEOSOME consists of 8 HISTONE PROTEINS around which DNA warps 1.65 times.
4. A CHROMATSOME consists of a NUCLEOSOME plus the H1 HISTONE.
5. The nucleosomes fold up to produce a 30-nm fibre…
6. … that forms LOOPS averaging 300nm in length.
7. The 300-nm fibres are COMPRESSED and FOLDED to produce a 250-nm-wide fibre.
8. TIGHT COILING of the 250-nm fibre produces the CHROMATID of a Chromosome.

Question 2

Look at a Model of a Chromosome.

Answer 2

Sister Chromatids
Centromere
One DNA molecule
2 DNA molecules
duplicated chromosome (2 DNA double helices)

Question 3

Centromere of the chromosome; how many? what is it? Crossovers?

Answer 3

1. At times, a chromosome consists of a single chromatid…
2. …at other times it consists of two (sister) chromatids.
3. The centromere is a constricted region of the chromosome where the KINETOCHORES form and the spindle microtubules attach.
4. The centromere regions show fewer CROSSOVERS during meiosis

Question 4

Telomere of the chromosome: what is a telomere? where?

Answer 4

1. The telomere regions have a special structure.
2. A telomere is the stabilising end of a chromosome; has the DNA sequence at the end of the chromosome.
   - G-rich single-stranded overhang
3. At the end of each telomere are many tandem repeat sequences.
4. Higher organisms have similar tandem repeats

Question 5

The Eukaryotic Cell Cycle?

G1, G0, G1/S Checkpoint, S, G2, G2/M Checkpoint, M, Spindle-Assembly Checkpoint, Cytokinesis and Transition to G1, Interphase. (10)

Answer 5

1. G1 (GAP 1) Phase:
   The cell grows in size during this phase.
2. G0 Phase:
   The cell may enter G0, which is a non-dividing phase where it remains in a quiescent state.
3. G1/S Checkpoint:
   After the G1 phase, the cell reaches a checkpoint called G1/S.
   At this checkpoint, the cell is assessed for proper conditions and DNA damage.
   If conditions are favourable and DNA is undamaged, the cell is committed to dividing.
4. S Phase (Synthesis):
   In the S phase, DNA duplication occurs.
   The cell’s genetic material is replicated to ensure each daughter cell receives a complete set of chromosomes.
5. G2 Phase:
   During the G2 phase, the cell prepares for mitosis.
   Additional growth and protein synthesis take place in preparation for division.

6.G2/M Checkpoint:
After the G2 phase, the cell encounters the G2/M checkpoint.
This checkpoint verifies the integrity of DNA replication and assesses if the cell is ready for division.
If everything is in order, the cell proceeds to enter mitosis.

7. M Phase (Mitosis):
   Mitosis is the phase of cell division.
   The cell undergoes nuclear division (karyokinesis) and cytoplasmic division (cytokinesis).
   The genetic material is evenly distributed to two daughter cells, each receiving a complete set of chromosomes.
8. Spindle-Assembly Checkpoint:
   After mitosis, there is a spindle-assembly checkpoint.
   This checkpoint ensures that the chromosomes are properly aligned and attached to the spindle fibres.
   Once this checkpoint is passed, cytokinesis occurs, completing the process of cell division.
9. Cytokinesis and Transition to G1:
   Cytokinesis involves the physical separation of the two daughter cells.
   After cytokinesis, the cell enters G1 phase again to begin another round of the cell cycle.
10. Interphase:
    Interphase comprises the period from the end of one cell division to the start of the next.
    It includes G1, S, and G2 phases, which collectively make up most of the cell cycle.

Question 6

Explain Cytokinesis in ANIMAL CELLS (2) vs PLANT CELLS (3)

Answer 6

IN ANIMAL CELLS
1. – Cytokinesis is accomplished by a CONSTRICTION OR GIRDLING.
2. – CENTRIOLES ARE VISIBLE in animal cells but invisible in plant cells

IN PLANT CELLS
1. – Cytokinesis is accomplished by the FORMATION OF THE CELL PLATE
2. – Cell plate: A NEW CELL WALL forms at the EQUATORILA PLATE
3. – The cell plate ENLARGES UNTIL IT COMPLETELY SEPARATES THE CELL INTO TWO CELLS.

Question 7

MITOSIS - What is the significance of Genetics? (4)

Answer 7

1. Keeps chromosome number constant
   – from cell to cell
2. Makes cloning possible – cloned
   – sheep, cattle, human? stem cells
   – vegetative propagation
3. Research in mitosis–cancer
4. agricultural production, etc.

Question 8

Explain the Process of Mitosis. IPMAT C?

Answer 8

1. Prophase:
   - Chromatin condenses into visible chromosomes.
   - The nuclear envelope disintegrates.
2. Metaphase:
   - Chromosomes align along the metaphase plate.
   - Spindle fibres form and attach to the centromere region of each chromosome.
3. Anaphase:
   - Sister chromatids separate and move to opposite poles of the cell.
   - Chromosomes migrate along the spindle fibres.
4. Telophase:
   - Chromosomes decondense into chromatin.
   - New nuclear envelopes form around each set of chromosomes.
   - The chromosomes have gathered into two clusters, one at the site of each future nucleus.
5. Cytokinesis:
   - Cytoplasm divides into two separate daughter cells.
   - Animal cells: Cleavage furrow forms and pinches off the membrane.
   - Plant cells: Cell plate forms and develops into a new cell wall.
6. Interphase.
   - Interphase is not technically a part of mitosis, but rather the period between successive rounds of mitosis.
   - It consists of three subphases: G1 phase, S phase, and G2 phase.
   - During interphase, the cell prepares for the next round of cell division.

Question 9

What is the Nuclear envelope, Chromatin, nucleolus, Centriole pairs, spindle pole, kinetochore, and spindle microtubules?

Answer 9

1. Nuclear Envelope:
   - Double membrane structure surrounding the nucleus.
   - Separates the nucleus from the cytoplasm.
   - Contains nuclear pores for molecular exchange.
2. Chromatin:
   - DNA, proteins, and RNA complex in the nucleus.
   - Loosely packed during interphase.
   - Condenses into visible chromosomes during cell division.
3. Nucleolus:
   - Structure within the nucleus.
   - Involved in ribosome production.
   - Contains RNA, DNA, and proteins.
4. Centriole Pairs:
   - Small cylindrical structures near the nucleus (in animal cells).
   - Organized in pairs.
   - Essential for spindle formation and chromosome separation during cell division.
5. Spindle Pole:
   - Centrosomes located at opposite ends of the cell.
   - Contain centrioles.
   - Organize and anchor spindle fibres during cell division.
6. Kinetochore:
   - Protein structure on the centromere of each chromatid.
   - Attachment point for spindle microtubules.
   - Important for chromosome movement and separation.
7. Spindle Microtubules:
   - Long protein filaments in the spindle apparatus.
   - Radiate from the spindle poles.
   - Assist in chromosome movement and separation during cell division.