Transcript DNA to Proteins to Natural Selection - Cal State LA

DNA to Proteins to Evolution
and Natural Selection
Biology 155
Krilowicz
Spring 2010
The Relationship between DNA and Proteins
Gene = a
segment of
DNA that
codes for a
single protein,
the sequence
of bases in the
DNA codes for
the sequence
of amino acids
in the protein
DNA cannot leave the
nucleus so an intermediate is
needed, mRNA (messenger
ribonucleic acid) serves as
the intermediate;
transcription is the process
by which DNA is copied
into mRNA
mRNA travels to the cytoplasm where ribosomal RNA (rRNA in
ribosomes) and transfer RNA (tRNA, binds amino acid and inserts
into protein at appropriate spot) are used to produce the protein coded
by the gene, the process is called translation or protein synthesis
Important Consequences
If the bases in the gene (DNA sequence) are
changed, then the messenger RNA base
sequence may change, which might change
the amino acid sequence in the protein
When the amino acid sequence of a protein
changes, the structure and function of the
protein might also change
What is Evolution?
Definition = genetic change through time =
change in the DNA nucleotide sequence
through time
ex. Change in species composition through
time as reflected in the fossil record
ex. Change in a single species through time
(antibiotic resistance in bacteria)
How does Evolution Occur?
A. Naturally occurring evolution has never
been observed
B. Note that natural selection is a theorized
mechanism that could lead to evolution. In
order for natural selection to operate the
following caveats must apply -
Caveats
1. Members of the species must show
variability in a trait
ex. Some bacteria are resistant to antibiotics,
while others are not
ex. What was the variable trait in the population
of beads studied in last week’s laboratory
simulation?
Caveats - continued
2. The variable trait must be genetically
determined
ex. Antibiotic resistant bacteria have a gene
that allows them to metabolize/detoxify the
antibiotic, while non-resistant bacteria lack
this gene
ex. Last week, we assumed that color of
beads was genetically determined.
Caveats - continued
3. There is a selective factor (ex. Climate,
predators, toxins, etc.) operative in the
environment that leads to selective
reproduction where some individuals leave
more offspring than others do
Examples
Breaks down antibiotic
Antibiotic resistant bacterium
Many offspring with trait
Does not break down antibiotic
Non-antibiotic resistant bacterium
Dies and leaves
no offspring
Who survived the laboratory predation simulation
and reproduced?
Caveats - continued
After many generations of selective
reproduction, the gene pool of the
population changes and is reflected in
observable characteristics
ex. The population of bacteria becomes
antibiotic resistant
ex. The populations became dominated by
camouflaged individuals.
NOTE: Evolution and natural
selection occur in a population
through time
Mutations (heritable changes in
the DNA) occur in individuals
Mutations can be –
a. Neutral = neither aids nor hinders an
individual’s survival at that time; common;
alters small segments of DNA, usually within a
single gene
b. Beneficial = increases the survival or ability of
an individual to reproduce; rare; alters small
segments of DNA, usually within a single gene
c. Lethal = eventually leads to an individual’s
death or inability to reproduce; common; alters
large segments of DNA, often across many
genes
For a sexually reproducing
species, the mutations must occur
in the gametes or cells involved
in gametogenesis to be passed on
to the offspring
What are the sources of genetic
variability in a population?
What does natural selection have to
work on?
Mutations
Occur in both asexually and sexually
reproducing populations; only source of
genetic variability for asexual species
Occur following exposure to certain
environmental factors such as UV light,
chemicals, radiation, etc.
Sexual reproduction increases
genetic variability in a population
1. Random shuffling of maternal and
paternal chromosomes during meiosis 1
when homologous chromosomes are
separated
ex. Humans = 223 = 8 million different
gametes
(23 = haploid # of chromosomes)
Fig. 8.16
Sources of genetic variability in a
sexually reproducing species continued
2. Crossing Over occurs during synapsis of
meiosis 1; bits of chromosomes are
exchanged between maternal and paternal
chromosomes
3. Random fertilization = random fusion of
an egg and a sperm cell
ex. 8 million eggs X 8 million sperms = 64
trillion possible offspring per human couple
Fig 8.18
Sources of genetic variability in a
sexually reproducing species continued
4. Random Mating = rare in animals
ex. Sea lion harems