Download Introduction to the Cell Cycle and Inheritance

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?
Human Genome
• 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)
Monohybrid Cross
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.
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?
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
Meiosis
Differences….
• 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
When do these events occur in meiosis?
Genetic reassortment also occurs during the
random fusion of gametes at fertilization
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)
A
Aa
A
AA
a
Aa
a
aa
aa
a
A
a
a
Aa
a
a
A
A
During which phase of meiosis are alleles segregated into gametes?
Hardy-Weinberg Model
p = frequency of allele A
q = frequency of allele a
… and p + q =1 (100%)
A
Aa
A
AA
A
Aa
a
a
aa
aa
a
A
A
a
a
Aa
a
a
A
AA
A
What are the values of p and q?
Hardy-Weinberg Model
p(A)
p(A)
eggs
q(a)
p2 (AA)
pq (Aa)
pq (Aa)
q2 (aa)
sperm
q(a)
p2 (AA) + 2pq (Aa) + q2 (aa) = 1
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
Solution
p2 (FF) + 2pq (Ff) + q2 (ff) = 1
•q2 = 20/25 = 80% = .80
•q = .80 = .89
ff
F_
•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?
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 mating
5. No natural selection
Causes of Microevolution
• Genetic drift
– Founder Effect
– Bottleneck Effect
• Gene flow
• Mutations
• Non-random mating
– Inbreeding
– Intra- or intersexual selection
• Natural selection
Which of these mechanisms produces new alleles?
Modes of Natural Selection