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 Antibiotic resistant bacterium Breaks down antibiotic Many offspring with trait Non-antibiotic resistant bacterium Does not break down antibiotic 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 = 2 23 = 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