Structure of chromosomes at prophase

Chromosomes are condensed, thick, and visible. Each consists of two sister chromatids joined at the centromere. The nuclear envelope starts breaking down.

What happens in G1, S, G2, and G0?

• G1 (Gap 1): Cell grows, makes proteins and organelles. 

• S (Synthesis): DNA is replicated → chromosomes become sister chromatids. 

• G2 (Gap 2): Final prep; makes spindle proteins; checks DNA. 

• G0: Resting/non-dividing phase; many cells stay here permanently (neurons). 

What does the mitotic spindle do?

• Organizes and moves chromosomes. 

• Attaches to kinetochores and pulls sister chromatids apart.

Stages of mitosis 

• Prophase: Chromosomes condense; spindle forms; nuclear envelope breaks. 

• Metaphase: Chromosomes line up in the middle. 

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

• Telophase: Chromatids arrive → nuclear envelopes reform → chromosomes uncoil.

What is a karyotype?

• A picture of all chromosomes arranged in homologous pairs. 

• Used to detect chromosomal abnormalities (e.g., Down syndrome). 

What is cohesin? 

A protein that holds sister chromatids together until anaphase

Telophase in plants vs animals 

• Animal cells: Cleavage furrow forms → cell pinches in two. 

• Plant cells: A cell plate forms in the middle → becomes the cell wall. 

7. A substance interfering with microtubules would… 

• Stop spindle formation, preventing chromosome separation. 

• Cell becomes stuck in metaphase → no proper division. 

(This is how chemo drugs like vinblastine work.) 

Cancer cell characteristics (vs normal cells) 

• Divide uncontrollably. 

• Do not respond to cell cycle checkpoints. 

• May undergo metastasis (spread). 

• Ignore density-dependent inhibition (grow on top of each other). 

Mitosis without cytokinesis results in… 

• One cell with two nuclei (multinucleated cell). 

Active MPF consists of…

yclin + Cdk (cyclin-dependent kinase) 
(MPF = Maturation Promoting Factor) 

What are homologous chromosomes?

• Chromosome pairs (one from each parent) with 

• same genes, 

• same size/shape, 

• but different versions (alleles). 

What happens in meiosis I vs meiosis II? 

Meiosis I (Reduction division) 

• Homologous chromosomes pair and separate. 

• Cell goes from diploid → haploid. 

• Crossing over and independent assortment occur. 

Meiosis II 

• Sister chromatids separate (like mitosis). 

• Produces 4 haploid cells. 

 

14. Differences between mitosis and meiosis 

Feature	Mitosis	Meiosis
Purpose	Growth/repair	Gamete formation
Divisions	1	2
Products	2 identical cells	4 genetically different cells
Chromosome #	Stays diploid	Becomes haploid
Crossing over	No	Yes

What is crossing over and when does it occur? 

• Exchange of DNA between non-sister chromatids in homologous pairs. 

• Occurs during Prophase I of meiosis. 

• Increases genetic diversity. 

 

How Mendel’s laws apply to meiosis 

• Law of Segregation: Alleles separate when gametes form → seen in anaphase I. 

• Independent Assortment: Homologous pairs line up randomly → metaphase I. 

Terms 

• Pleiotropy: One gene affects many traits (ex: sickle-cell). 

• Epistasis: One gene hides or controls another gene (ex: lab coat color). 

• Multiple alleles: More than two alleles exist (ex: ABO blood group). 

• Polygenic: Many genes contribute to one trait (ex: height, skin color). 

Inheritance of blood types & Rh

• ABO: Multiple alleles + codominance. 

• IA and IB are codominant. 

• i is recessive. 

• Rh factor: Simple dominant. 

• Rh⁺ = dominant 

• Rh⁻ = recessive 

What is phenylketonuria (PKU) and treatment? 

• A genetic disorder where the body cannot break down phenylalanine. 

• Can cause brain damage if untreated. 

• Treated with: 

• Low-phenylalanine diet, especially in childhood. 

Be able to use pedigrees to determine inheritance patterns.

• Autosomal dominant: Appears every generation; males/females equally. 

• Autosomal recessive: Skips generations; carriers present. 

• X-linked recessive: Mostly males; passed from carrier mothers. 

• X-linked dominant: Affected fathers pass to all daughters.