Cell Cycle Pre-Reading

What is the typical duration and significance of the skin cell cycle?

• Skin cells divide frequently

• Cycle lasting approximately 12–24 hours

• This rapid turnover maintains epidermal integrity and facilitates wound healing

Describe the cell cycle characteristics of liver cells (hepatocytes)

• Liver cells undergo transient withdrawal from the cell cycle under normal conditions

• They retain the ability to reassemble their cell cycle control system

• They typically divide once every year or two

• Can rapidly re-enter the cycle for regeneration after liver injury

What is unique about mature nerve cells and muscle cells in terms of division?

• Mature nerve and muscle cells are terminally differentiated

• They do not divide after maturity

• They remain permanently in the G₀ state, a quiescent phase outside the active cycle, explaining why nerve and muscle damage are hard to repair

What is the behavior of certain lymphocytes regarding the cell cycle?

Some lymphocytes can withdraw from and re-enter the cell cycle repeatedly in response to immune stimuli and physiological demands

What are the main structural phases of the cell cycle?

The cell cycle consists of three main parts:

• Interphase

• Mitotic Phase (M phase)

• Quiescent Phase (Senescence)

What proportion of time do cells spend in interphase?

90%

What are the subphases of interphase?

Interphase includes the G₁ phase, S phase, and G₂ phase

What occurs during the G₁ phase?

G₁ is characterized by:

• Cellular growth and metabolic activity

• Recovery from previous division

• Organelle duplication

• Cell enlargement

• Accumulation of materials for DNA synthesis, and RNA and protein synthesis

What happens in the S phase?

• DNA replication occurs, producing two identical sister chromatids

• RNA and protein synthesis continue

What happens in the G₂ phase?

• G₂ occurs between DNA replication and the onset of mitosis

• Proteins required for cell division are synthesised

What defines the G₀ phase?

• G₀ is a quiescent state in which cells are metabolically active but not dividing

• Some cells are permanently in G₀ (senescent), while others can re-enter the cycle

What causes most variation in cell cycle length?

Variations are mainly due to differences in G₁ duration or time spent in G₀

What is the typical duration of the S phase through M phase in mammalian cells?

12–24 hours

What is mitosis?

Mitosis is nuclear division in eukaryotic cells producing two genetically identical daughter cells

What key events occur during prophase?

• Nucleoli disappear as rRNA synthesis stops

• Chromatin condenses into visible chromosomes of two sister chromatids joined at the centromere

• The mitotic spindle forms from microtubules between centrosomes

• Centrosomes move toward opposite poles

What defines prometaphase?

• The nuclear envelope fragments completely

• Spindle fibres extend from poles toward the equator

• Kinetochores form at each chromatid’s centromere

• Kinetochore microtubules attach to kinetochores

What characterizes metaphase?

• Centrosomes are at opposite poles

• Chromosomes align at the metaphase plate

• Centromeres are precisely aligned

• The mitotic spindle includes kinetochore and non-kinetochore fibres

• Each chromatid is attached to opposite spindle poles

• Proper alignment is essential for accurate chromosome segregation

What happens during anaphase?

• Anaphase A: Sister chromatids separate at centromeres and move to opposite poles

• Anaphase B: Spindle poles move apart; non-kinetochore microtubules elongate, stretching the cell

What events occur during telophase?

• Non-kinetochore microtubules continue elongating

• Daughter nuclei form at spindle poles

• Nuclear envelopes reform

• Nucleoli reappear

• Chromatin decondenses, making chromosomes less distinct

What is cytokinesis and how does it occur?

Cytokinesis divides the cytoplasm and organelles using a contractile ring of actin and myosin filaments, cleaving the cell into two daughter cells

What is meiosis?

• Meiosis is a specialized division that produces four genetically diverse haploid gametes from one diploid cell

• Fundamental to sexual reproduction and genetic diversity

What happens before meiosis during interphase?

• Each chromosome replicates, forming two sister chromatids joined at the centromere

• Two centrosomes (each with centrioles) are present

• The cell prepares for meiotic division

Why is Prophase I significant?

• It is the longest and most complex meiotic stage, lasting about 90% of meiosis

• Chromosomes condense, homologous chromosomes pair (synapsis) forming tetrads, and crossing over occurs

What is synapsis?

Synapsis is the pairing of homologous chromosomes forming a tetrad (bivalent), composed of two homologous chromosomes (four chromatids total)

What happens during crossing over?

• Non-sister chromatids exchange segments at chiasmata

• Producing genetic recombination and new allele combinations

• Increasing offspring diversity

What defines Metaphase I?

• Tetrads align at the metaphase plate

• Homologous pairs stay attached at chiasmata

• Sister kinetochores face the same pole

• Homologous centromeres face opposite poles

• Independent assortment occurs, randomising chromosome orientation and promoting diversity

What happens in Anaphase I?

• Spindle fibers contract, pulling homologous chromosomes to opposite poles

• Sister chromatids remain attached

• Homologs separate, reducing chromosome number from diploid to haploid

• The kinetochore orientation differs from mitosis, as both sister chromatids move together to the same pole

What occurs during Telophase I and cytokinesis?

• Each pole gets a haploid set of chromosomes (each with two chromatids)

• Cytokinesis produces two haploid daughter cells

• Spindle may or may not fully disassemble.

• An interkinesis period may occur (no DNA replication)

• Nuclear envelopes may briefly reform

How does Meiosis II compare to mitosis?

Meiosis II resembles mitosis because sister chromatids separate

What happens in each stage of Meiosis II?

• Prophase II: Chromosomes condense; new spindles form

• Metaphase II: Chromosomes align at the plate; kinetochores orient toward opposite poles

• Anaphase II: Sister chromatids separate and move to opposite poles

• Telophase II: Nuclear envelopes reform; cytokinesis produces four haploid gametes

What is non-disjunction?

• Non-disjunction is the failure of homologous chromosomes (in Meiosis I) or sister chromatids (in Meiosis II or mitosis) to separate properly

• Causing gametes with abnormal chromosome numbers (aneuploidy)

What are the consequences of non-disjunction?

• When abnormal gametes fertilise, the resulting zygotes have chromosomal imbalances

• Potentially leading to developmental disorders or non-viable embryos