Comprehensive and detailed Exam 1 Study guide for Bio 142.
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Section 16.2: Central Dogma of Molecular Biology
Genetic code hypothesis: codons may code for amino acids in genes
RNA (mRNA) links genes (DNA) and proteins
o This was discovered because DNA is in nucleus and ribosomes are outside the
nucleus. So there has to be something to link the two together
RNA polymerase polymerizes ribonucleotides into RNA
RNA polymerase synthesizes RNA molecules according to info provided by sequence
of bases in particular stretch of DNA
RNA polymerase DOES NOT require a primer
Genes code for proteins indirectly: sequence of bases in DNA specify sequence of
bases in RNA which specifies sequence of amino acids in proteins
Transcription: copying DNA hereditary info in DNA to RNA
Translation: using info in nucleic acids to create proteins
Genotype is determined by sequence of bases
phenotype is product of the proteins genes create
Exceptions to central dogma include that
o Genes code for RNA molecules that do not function as mRNAs and aren’t
translated into proteins
o Info sometimes flows from RNA back to DNA
How does info flow from DNA to RNA? Some viruses have RNA. When RNA viruses
infect a cell, reverse transcriptase synthesizes a DNA from the virus’ RNA. This
happens in HIV
Section 16.3: Genetic Code
The three-base triplet code was discovered when Gamow reasoned that
a) One base (C, G, A, T) wouls only code for 4 amino acids
b) Two bases would only code for 16 amino acids
(4 x 4)
c) Three bases provides 64 different amino acids,
which is way more than 20
Start codon = AUG
Stop codons = UAA, UAG, UGA
This coding for amino acids is redundant,
unambiguous, non-overlapping, universal, and
conservative
Mutation affecting the third letter in codon are less
likely to change the amino acid and cause mutations
Section 16.4: How Mutation Can Modify Genes and
Chromosomes
Mutation: any permanent change in organisms DNA. Change in genotype.
Point mutation = single base change. There are four kinds:
o Missense: mutation causes change in amino acid sequence and phenotype
(color change in rats)
1
, Grace Essien BIOL 141
o Silent: mutation doesn’t change the kind of amino acid
o Frameshift: insertion or deletion
o Nonsense: regular codon becomes a stop codon, resulting in a shorter
polypeptide/protein
Point mutation occur in DNA sequences that don’t code for proteins. They’re not
classified in any of the categories above because those terms apply only to
mutations that can affect gene expression
In terms of their impact on organisms, biologists divide mutations into three
categories:
o Beneficial: increasing fitness of organism (missense)
o Neutral: having no effect on fitness (silent)
o Deleterious: harmful to fitness (sometimes missense, see pg. 313), probably
frameshift and nonsense too)
Chromosome mutations occur also. Chromosome mutation can be beneficial, neutral
and deleterious as well. Polyploidy is increase in chromosomes and aneuploidy is an
insertion or deletion
The composition of chromosomes can change in three ways
o Inversion: accidental breaks in chromosomes cause segments of DNA to be
flipped
o Translocation: accidental breaks in chromosomes are relocated to another
chromosome
o Deletion: segment of chromosome is lost
o Duplication: additional copies of segments are present
Chromosomes in cancer cells exhibit deleterious chromosome mutations (aneuploidy,
inversions, translocations, deletions, and duplications
Karyotype: complete set of chromosomes in a cell
SUMMARY: point and chromosome mutations are random chnges in DNA that can
produce new genes, new alleles, and new traits
o Individually, mutations can cause disease, death and lead to increased fitness
o Population wise, mutations can lead to heritable variation
There are three principles to keep in mind when predicting the sequence of the mRNA
produced by transcription of a particular DNA sequence.
a) The RNA polymerase reads the sequence of DNA bases from only one of the two
strands of DNA: the template strand.
b) The RNA polymerase reads the code from the template strand in the 3' to 5' direction
and thus produces the mRNA strand in the 5' to 3' direction.
c) In RNA, the base uracil (U) replaces the DNA base thymine (T). Thus the base-pairing
rules in transcription are A→U, T→A, C→G, and G→C, where the first base is the
coding base in the template strand of the DNA and the second base is the base that
is added to the growing mRNA strand.
Extra Notes
Addition plus deletion create double mutants that cancel each other out, nut there
will still be a difference that won’t affect the creation of the protein
Three additions and three deletions can be combined to restore the function of the
cell
CHAPTER 17: How Genes Work
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