biology( cell cycle)

What is the cell cycle?

The sequence of events by which a cell grows, replicates its DNA, and divides into two daughter cells.

What are the two major parts of the cell cycle?

Interphase and M phase (mitosis + cytokinesis

Which phase occupies most of the cell cycle?

Interphase

What are the three stages of interphase?

G₁, S, and G₂

What happens during G₁?

Cell growth, protein synthesis, RNA synthesis, organelle duplication, and normal metabolism.

What happens during S phase?

DNA replication occurs, producing sister chromatids.

What happens during G₂?

Further growth, synthesis of mitotic proteins, and DNA quality control.

During which phase is DNA replicated?

S phase only

Does chromosome number double during S phase?

No. DNA content doubles, but chromosome number remains the same.

Difference between chromosome number and DNA content after S phase?

Chromosome number stays the same; each chromosome now consists of two sister chromatids, so DNA content doubles.

What is checked at the G₁ checkpoint?

Cell size, nutrients, growth factors, and DNA damage.

What is G₀?

A non-dividing but metabolically active state that some cells enter.

What is checked at the G₂ checkpoint?

Completion and accuracy of DNA replication.

What is checked at the M (spindle) checkpoint?

Proper attachment of all chromosomes to spindle fibers before anaphase.

What are cyclins?

Regulatory proteins whose levels rise and fall to control cell-cycle progression.

What are CDKs?

Cyclin-dependent kinases activated by cyclins to drive the cell cycle forward.

Why is p53 called the “guardian of the genome”?

It pauses the cell cycle, promotes DNA repair, or triggers apoptosis if DNA damage is irreparable.

What is a tumor suppressor gene?

A gene that slows or stops cell division and helps prevent cancer.

What is a proto-oncogene?

A normal gene that promotes cell growth; if mutated, it can become an oncogene that drives uncontrolled cell division.

What is the purpose of mitosis?

To produce two genetically identical diploid daughter cells for growth and tissue repair.

What is cytokinesis?

Division of the cytoplasm to produce two separate daughter cells.

Name the stages of mitosis.

Prophase, Metaphase, Anaphase, Telophase (PMAT).

What happens during prophase?

Chromosomes condense, spindle forms, nuclear envelope breaks down, nucleolus disappears.

What happens during metaphase?

Chromosomes align individually at the metaphase plate; spindle fibers attach to kinetochores.

What happens during anaphase?

Sister chromatids separate and move to opposite poles.

What happens during telophase?

Nuclear envelopes reform, chromosomes decondense, nucleoli reappear.

How many daughter cells does mitosis produce?

2

Are daughter cells from mitosis genetically identical?

Yes

What are the main functions of mitosis?

Growth, tissue repair, cell replacement, and asexual reproduction.

What is apoptosis?

Programmed cell death.

Why is apoptosis important?

It removes damaged, infected, or unnecessary cells and helps prevent cancer.

Difference between apoptosis and necrosis?

Apoptosis is programmed and usually does not cause inflammation; necrosis is uncontrolled and often causes inflammation.

What is the purpose of meiosis?

To produce genetically different haploid gametes.

Where does meiosis occur?

Germ cells in the testes and ovaries.

How many cell divisions occur in meiosis?

2

How many DNA replications occur in meiosis?

One, before Meiosis I.

What separates during Meiosis I?

Homologous chromosomes.

What separates during Meiosis II?

Sister chromatids.

Why is Meiosis I called the reductional division?

Because it halves the chromosome number by separating homologous chromosomes.

What are homologous chromosomes?

Chromosomes carrying the same genes, one maternal and one paternal, which may have different alleles.

Difference between homologous chromosomes and sister chromatids?

Homologous chromosomes come from different parents and may differ in alleles; sister chromatids are identical copies formed during DNA replication.

What is synapsis?

Pairing of homologous chromosomes during Prophase I.

What is a tetrad (bivalent)?

A pair of homologous chromosomes containing four chromatids.

What is crossing over?

Exchange of DNA between non-sister chromatids of homologous chromosomes during Prophase I.

What are chiasmata?

The visible points where crossing over occurs.

Why does crossing over increase genetic variation?

It creates new combinations of alleles on chromosomes.

What is independent assortment?

Random orientation of homologous chromosome pairs during Metaphase I.

Why does independent assortment increase genetic variation?

It produces different combinations of maternal and paternal chromosomes in gametes.

Name the three major sources of genetic variation.

Crossing over, independent assortment, and random fertilization.

How many chromosomes do human body cells have?

46 (2n).

How many chromosomes do human gametes have?

23 (n).

After S phase, how many chromosomes and chromatids does a human cell contain?

46 chromosomes and 92 chromatids.

After Meiosis I, how many chromosomes and chromatids are in each daughter cell?

23 chromosomes and 46 chromatids

After Meiosis II, how many chromosomes and chromatids are in each gamete?

23 chromosomes and 23 chromatids.

What is nondisjunction?

Failure of chromosomes or sister chromatids to separate correctly during meiosis.

What is the result of nondisjunction?

Aneuploidy (abnormal chromosome number).

Cause of Down syndrome?

Trisomy 21 caused by nondisjunction.

Karyotype of Turner syndrome?

45,X.

Karyotype of Klinefelter syndrome?

47,XXY.

Prophase vs Prophase I.

• Prophase: Chromosomes condense.

• Prophase I: Synapsis and crossing over also occur.

Metaphase vs Metaphase I.

• Metaphase: Individual chromosomes align.

• Metaphase I: Homologous pairs align.

Anaphase vs Anaphase I.

• Anaphase: Sister chromatids separate.

• Anaphase I: Homologous chromosomes separate.

Why does mitosis not create genetic variation?

No crossing over or independent assortment occurs.

DNA is damaged after replication. Which checkpoint detects this?

G₂ checkpoint.

During which stage does chromosome number first become haploid?

At the end of Meiosis I, after homologous chromosomes have separated.

During which stage do sister chromatids become individual chromosomes?

Anaphase (mitosis) and Anaphase II (meiosis).

Explain why meiosis produces genetic variation.

Meiosis produces genetic variation through crossing over during Prophase I, independent assortment during Metaphase I, and random fertilization, generating unique combinations of alleles in every offspring.