Introduction to the Cell Cycle and Inheritance

DNA - review• DNA is the genetic

material in all organisms• The units of genetic

information are genes• A cell’s DNA controls

what proteins are made and in this way controls most of the cell’s activity

• The structure of DNA molecules enables exact copies to be made How is the function of the cell

in the center different from the other cells in this picture?

Cell Reproduction

• a characteristic of all living things

• for maintenance of the species

• for growth and repair

How is binary fission similar to mitosis in eukaryotic cells?How is it different?

Cell Division: the basics• Two types of nuclear division

– Mitosis– Meiosis

• Before cytokinesis, replicated chromosomes must be distributed precisely into two new nuclei (daughter cells)– Mitosis produces two nuclei that contain the same

genetic information as the original nucleus (clones)

– Meiosis produces new nuclei with only half the number of chromosomes

Do all human cells divide at the same rate?

Eukaryotic Chromosomes• In the somatic (body)

cells of most plants and animals, chromosomes exist in homologous pairs (diploid = 2N)

• Homologous chromosomes are morphologically and genetically similar – contain the same genes,

but not necessarily the same form of the genes

What is chromatin and where would you find it?

• 46 chromosomes in 23 pairs

• 22 pairs of autosomes

• 1 pair of sex chromosomes– Females XX– Males XY

How many of chromosome #3 will there be in a human gamete?

How many sex chromosomes will there be in a human gamete?

How is genetic sex inherited? (Draw a Punnett square)

Human Genome

Monohybrid Cross

a)  Predict the percentage of offspring expected to have short whiskers from the cross of two long-whiskered seals, one that is homozygous dominant and the other that is heterozygous?

b) If the genotype of one parent seal is WW and the other is ww, what percent of offspring is predicted to have the short whiskers?

In seals, the gene for the length of the whiskers has two alleles.  The dominant allele (W) codes for long whiskers & the recessive allele (w) codes for short whiskers.

Cell Cycle

• 4 distinct periods

What does the cell spend most of its life doing?

Do you think DNA synthesis is an “expensive” process? Why or why not?

Mitosis is a continuous process described in 5 phases:

Early Mitosis

Late Mitosis

Cytokinesis

How are organelles like mitochondria distributed to daughter cells?

Meiosis

•Many eukaryotic organisms reproduce sexually.

•Some organisms can reproduce either by sexual or asexual means.

What circumstances might favor asexual reproduction? …sexual reproduction?

Meiosis

Similarities…

• DNA replication before the start of cell division

• Spindle forms and chromosomes move to opposite poles of the cell

• Cytokinesis when nuclear division is complete

MeiosisDifferences….• Meiosis has two successive nuclear divisions

w/o DNA replication during the interphase between

• During Meiosis I, chromosomes behave as homologous pairs rather than individual units

• Meiosis results in 4 daughter nuclei rather than 2

• The purpose of meiosis is to produce genetically different offspring via fusion of gametes

Overview of Meiosis

How many chromosomes are represented here?

How many homologous pairs of chromosomes begin meiosis for the production of gametes in humans?

Why are homologous pairs during metaphase I called “tetrads”?What event has happened during meiosis I that created a new gene

combination?

Genetic Variation

Meiosis produces genetic reassortment in two ways:

• Crossing over

• Independent assortment

Genetic reassortment also occurs during the random fusion of gametes at fertilization

When do these events occur in meiosis?

Modern Synthetic Theory

• Understanding the process of natural selection requires hereditary processes that Darwin could not explain

• Mendel’s principles of inheritance went unnoticed until the early 1900’s

• The science of population genetics (1930’s) emphasized the importance of populations (and not individuals) as units of evolution

The Genetic Structure of a Population

• Species = group of similar organisms with the potential to interbreed successfully

• Population = localized group of organisms belonging to the same species

• Gene pool = the total set of alleles in a population Do the events in meiosis and

random fertilization cause evolution?

• Segregation of alleles into gametes and recombination during fertilization will not change the frequency of alleles in a gene pool – the gene pool will remain stable (in equilibrium)

Aa

Aa

Aa

aa

aa

AA

aa

a

a

a

a

a

A

A

A

A

A

During which phase of meiosis are alleles segregated into gametes?

Aa

Aa

Aa

aa

aa

AA

aa

a

a

a

a

a

A

A

A

A

A

Hardy-Weinberg Modelp = frequency of allele A

q = frequency of allele a… and p + q =1 (100%)

AA

A

A

What are the values of p and q?

Hardy-Weinberg Model

p(A) q(a)

p(A)

q(a)

p2 (AA) + 2pq (Aa) + q2 (aa) = 1

eggs

sperm

p2 (AA) pq (Aa)

pq (Aa) q2 (aa)

Using the Hardy-Weinberg equation:

In our lab, 20 out of 25 people had connected earlobes, a trait controlled by a single recessive allele. Estimate the frequency of the connected (f) and free (F) alleles in this population, as well as the frequency of the diploid genotypes.

p2 (FF) + 2pq (Ff) + q2 (ff) = 1

Solutionp2 (FF) + 2pq (Ff) + q2 (ff) = 1

•q2 = 20/25 = 80% = .80

•q = .80 = .89

•p = 1 - q = 1 - .89 = .11

•FF(p2) = .01 = 1%

•Ff (2pq) = .20 = 20%

•ff (q2) = .79 = 79%

How is this equilibrium model useful to biologists?

ff F_

5 requirements for H-W equilibrium:1. Large population size

• decreasing the likelihood that chance events will change allele frequencies i.e. no genetic drift

2. No gene flow (migration)How might alleles move between populations?

3. No net mutations (new alleles)4. Random mating5. No natural selection

Causes of Microevolution• Genetic drift

– Founder Effect– Bottleneck Effect

• Gene flow• Mutations• Non-random mating

– Inbreeding– Intra- or intersexual selection

• Natural selectionWhich of these mechanisms produces new alleles?

Modes of Natural Selection