Lecture 1 - Mitosis, Meiosis, and Monohybrid Crosses

Mitosis

Clonal replication of chromosomes followed by division of chromosomes into two cells (forms somatic cells)

Meiosis

Replication and pairing of homologous chromosomes that undergo two rounds of division forming four cells that are NOT identical (forms gametes)

What are characteristics of bacterial DNA

1. one large circular chromosome - Double stranded DNA molecule that is NOT contained in a nucleus

2. Additional small plasmids

3. Chromosomes replicates by fission

4. NO meiosis

Clone

A population of genetically identical cells

Colony

A visible mass of cells

Define fission and describe its steps

Fission - A mother cell divides to produce two daughter cells

1. The mother cell’s chromosome is duplicated prior to fission

2. Each daughter cell receives one copy of the chromosome

What are two main characteristics of eukaryotes

1. Chromosomes are contained in the nucleus

2. Conspicuous organelles with a chromosome that replicates by fission

What are the steps to the cell cycle

1. G1 - Growth and metabolism

2. S - DNA synthesis and chromosome duplication

3. G2 - Preparation for mitosis

4. Division - Prophase, metaphase, anaphase, telophase, cytokinesis

What are the stages of mitosis?

1. Prophase

2. Metaphase

3. Anaphase

4. Telophase

Prophase

As a cell enters mitosis, its duplicated chromosomes condense into rod shaped bodies

Metaphase

Chromosomes migrate to the middle plane of the cell

Anaphase

The centromere that holds the sister chromatids of duplicated chromosomes together SPLITS

Sister chromatids are separated

Telophase

Chromosomes decondense and the nuclear membrane reforms around them (Each daughter cell produced by mitosis are genetically similar)

How does meiosis affect the number of chromosomes?

Chromosome number is reduced

• 2N → 1N

• 2N = diploid

• 1N = haploid

What do fused gametes represent?

Has correct diploid composition of chromosomes

Meiosis I

Maternal and paternal homologous chromosomes separate into separate cells

Meiosis II

Chromosome duplicates (sister chromatids) separates

Describe independent assortment of chromosomes (Meiosis)

• 50/50 chance that a gamete will get paternal or maternal chromosomes

• 2 possibilities for each of the 23 chromosomes in gamete

• 2²³ - 8 million chromosomes

Describe crossing over/recombination (meiosis)

• Occurs during prophase I

• Portions of two chromatids are exchanged

• Can result in maternal and paternal genes being shared on the same chromosome

Describe random fertilization

Outcome is that genes and their alleles are brought together with formation of zygote in LOTS of unique combinations (from meiosis)

NOT possible with clonal reproduction

Egg and sperm - 1 of 8 million combinations

zygote - 1 of 64 trillion diploid combinations

Why is generating new combinations of genes good?

• Meiosis and fertilization generates new combinations of genes in the next generation that may be adaptive

• Brings together deleterious recessive alleles that expose the trait they encode to natural selection

• Mutation generates genetic variation

What are three advantages of working on sweet peas?

1. Many varieties with observable alternate versions of traits

2. Self pollination but can easily cross pollinate

3. True breeding varieties (strains) self pollination over many generations produce strains that are homozygous at all genes

Define monohybrid cross

Sperm cells associated with one trait crossed with egg cells associated with paired trait

Describe traits shown in F1 and F2 generations

• F1 hybrids exhibit only one parental trait (dominant trait)

• F2 hybrids exhibit EITHER parental trait (dominant or recessive)

• F2 hybrids ratio - 3:1 dominant : recessive

Gene

Determines a trait

Locus

A discrete/specific position of a gene on a chromosome

Alleles

Different forms of the gene at a given locus (tall vs short allele)

Phenotype

observable, physical appearance

Genotype

The genetic makeup

What is Mendel’s principle of dominance and segregation?

The phenotype of one allele can dominate or mask the phenotype of its homologous recessive allele

What happens to the gamete formation during meiosis?

The two alleles on two separate homologous chromosomes (paternal and maternal) are separated → Each gamete receives ONE allele (50% chance of gett