I. Control of Cell Cycle

• Normal growth & development require control of cell cycle (timing, location)

• Some cells divide frequently (Ex: Skin cells)• Some cells divide for repair (Ex: Liver cells)• Some cells don’t divide (Ex: Nerve cells)

A. Checkpoints• Control points in cell cycle• Three major checkpoints

• G1

• G2

• M

Fig. 12.15

I. Control of Cell CycleA. Checkpoints

• “No” at G1 checkpoint directs cell to G0

• Most cells in human body in G0 phase

Fig. 12.16

II. Sexual Life Cycles

• Organisms with sexual life cycles typically contain two copies of each chromosome

• Two basic cell types1) Somatic cells

• Diploid (2n) – Two complete sets of chromosomes• Chromosome set includes n-1 autosomes & 1 sex

chromosome• Pairs of autosomes = homologous pairs (same loci)• Females – homologous sex chromosomes

2) Gametes• Haploid (n)• Sperm cells, ova• Produced by meiosis• Unite in fertilization to produce diploid zygote

• Sexually reproducing organisms alternate between diploid & haploid stages

Fig. 13.5

Fig. 13.6

III. Meiosis

• Similar to mitosis, except DNA replicated once before cells divide twice

• Divisions – Meiosis I, Meiosis II

Fig. 13.7

Fig. 13.8

Long, Complex Phase

>90% of meiosis (days)

Synapsis, Synaptonemal Complex

Fig. 13.8

Fig. 13.9

Fig. 13.9

Fig. 13.10

Fig. 13.9

Fig. 13.11

IV. Principles of Inheritance

A. Historical Background• First person to study inheritance quantitatively

was Gregor Mendel (1822-1884)• Selective breeding common• Known that

1) Hybrid plants with same two parents are similar

2) Mating of hybrid offspring produces diverse progeny (primary hybrids don’t breed true)

IV. Principles of Inheritance

B. Mendel’s Experiments• Used garden pea (Pisum sativum)

1) Easy to grow

2) Many varieties readily available

3) Flowers could be pollinated in a controlled manner

Fig. 14.2

IV. Principles of Inheritance

B. Mendel’s Experiments• Before experiments, Mendel developed true-

breeding lines with consistent phenotypes• Used characteristics that could be studied easily• Strengths of approach

1) Clearly-defined traits

2) Simple experiments

IV. Principles of Inheritance

B. Mendel’s Experiments• Prevailing thought: Traits controlled by fluids

that blended together in offspring• Mendel crossed parent plants (P generation)

with different traits• Produced offspring resembling only one parent in first

generation (F1 generation)

• Crossed two F1 hybrids• Produced offspring (F2 generation) resembling both

grandparents

Fig. 14.3

IV. Principles of Inheritance

B. Mendel’s Experiments• Prevailing thought: Traits controlled by fluids

that blended together in offspring• Mendel crossed parent plants (P generation)

with different traits• Produced offspring resembling only one parent in first

generation (F1 generation)

• Crossed two F1 hybrids• Produced offspring (F2 generation) resembling both

grandparents

• Results suggested existence of heredity factors (genes) with different forms (alleles)

• Dominant trait masked recessive trait in F1 generation

Fig. 14.4

IV. Principles of Inheritance

C. Mendel’s Model• Developed hypothesis to explain inheritance

patterns1. Alternative versions of genes account for

variations in inherited characters2. For each character, an organism inherits two

alleles, one from each parent3. If two alleles at a locus differ, dominant allele

determines phenotype; recessive allele doesn’t affect phenotype

4. Law of segregation: Alleles separate during gamete formation; one allele in each gamete (mechanism?)

• Reported findings in 1866; rediscovered in 1900