Chapter 02: Mitosis and Meiosis

What is chromatin?

DNA wrapped around histone protein forming nucleosomes; loosely packed during interphase and condensed into chromosome during division.

What is a chromosome?

A highly condensed DNA molecule visible during cell division; contains genes and has a centromere.

What is the centromere?

Region where sister chromatids attach and where the kinetochore forms; determines chromosome movement.

What is a kinetochore?

Protein complex assembled at the centromere that attaches chromosomes to spindle microtubules

Define sister chromatids

Two identical DNA copies formed during S phase and joined at the centromere

Define homologous chromosomes

A maternal and paternal pair with same genes at same loci but possibly different alleles

What is diploid (2n)?

Cells with two sets of chromosomes (humans: 46).

What is haploid (n)?

Cells with one set of chromosomes (gametes: 23 in humans).

Phases of cell cycle?

G1 → S → G2 → M (mitosis) → cytokinesis

What happens in S phase?

DNA replication; chromosome duplicated into sister chromatids.

What happens in G2?

Cell growth and prep for mitosis; DNA already replicated.

Mitosis purpose?

Produces 2 genetically identical diploid daughter cells.

Stages of mitosis?

Prophase → Metaphase → Anaphase → Telophase → Cytokinesis

Key events in prophase?

Chromosome condense; nuclear envelope breaks down; spindle forms.

Key events in metaphase?

Chromosomes align at metaphase plate.

Key events in anaphase?

Sister chromatids separate and become independent chromosomes.

Key events in telophase?

Nuclear envelopes reform; chromosomes decondense.

When does chromosome number double?

Only during anaphase (because sister chromatids separate).

When is chromosome number reduced?

During cytokinesis after separation is complete.

Meiosis purpose?

Produces four genetically different haploids gametes

Meiosis I vs. Meiosis II?

Meiosis I: homologous chromosomes separate (reductional division). Meiosis II: sister chromatids separate (like mitosis)

What is synapsis?

Pairing of homologous chromosome in prophase I

What is a tetrad?

Structure of paired homologous chromosomes (4 chromatids)

What is crossing over?

Exchange of DNA between non-sister chromatids in prophase I.

What is independent assortment?

Random orientation of homologous pairs at metaphase I

What is nondisjunction?

Failure of chromosome to separate properly → abnormal chromosome number.

Difference between spermatogenesis and oogenesis

Spermatogenesis: 4 functional sperm. Oogenesis: 1 functional egg + polar bodies.

Why is meiosis important for diversity?

Crossing over + independent assortment → massive allele combo (2²³ possible combo in humans)

Why does chromosome number temp. double in anaphase but not reflect ploidy change?

Because sister chromatids separate and each gains it own functional centromere, they are counted as independent chromosomes. However, ploidy (number of homologous sets) does not change.

Explain how crossing over increases genetic variation at the molecular level.

Non-sister chromatids exchange DNA segments, creating recombinant chromosomes containing novel allele combinations that did not exist in either parent.

Why is meiosis I considered reductional division?

Homologous chromosomes separate, reducing chromosome number from 2n to n. Sister chromatids remain attached.

Compare metaphase in mitosis vs. metaphase I in meiosis.

Mitosis: individual chromosome align. Meiosis I: homologous pairs (tetrads) align, enabling independent assortment.

How does nondisjunction in meiosis I differ from meiosis II?

Meiosis I: homologous chromosomes fails to separate → all gametes abnormal. Meiosis II: sister chromatids fail to separate → ½ of gametes abnormal.

Why does independent assortment follow Mendel’s Law of Independent Assortment?

Random orientation of homologous pairs during metaphase I causes alleles of different genes ( on different chromosomes) to segregate independently

Why must DNA replication occur only once before meiosis?

If replication occurred twice, chromosome number would not be reduced properly, disrupting ploidy and genome stability.

Explain the relationship between number of DNA molecules and number of chromatids.

Each chromatid equal one DNA molecule. Therefore, DNA molecule number equals chromatid number.

Why does oogenesis produce unequal cytoplasmic division?

To concentration cytoplasm, nutrients, and organelles into one viable egg capable of supporting early embryonic development

Why is genetic variation from meiosis essential for population survival?

Variation increases adaptability to environmental changes, enhances evolutionary potential, and reduces susceptibility to uniform disease vulnerability.