Biology 155 Study Guide – This is only a guide to help you study.
You are ultimately responsible for everything in the book chapters
covered and everything discussed in lecture.
Know that a gene is a code for a protein.
Know that the primary structure of proteins is encoded in the nucleotide sequence of
Know that synthesis of proteins using the DNA code is a two step process
o Transcription - synthesis of messenger RNA (mRNA) using one of the DNA
strands as a template
o Translation - synthesis of a polypeptide using the nucleotide sequence of mRNA
Know that the flow of information from DNA to RNA to protein is called "The Central
Dogma of Molecular Biology."
Know the steps in transcription: initiation at the promoter, elongation of mRNA,
termination at the stop sequence.
Know that the genetic code is a triplet code, there are 64 codons, 3 codons are stop
codons, the remaining codons code for 20 amino acids.
Know the steps in translation:
o initiation: fmet-tRNA, initiation factors, small ribosomal subunit, + large ribosomal
o elongation: growing polypeptide found at P site, each new amino acid enters at A
o translocation: movement of the ribosome along the tRNA one codon at a time
o termination: release factor recognizes stop codon and cleaves polypeptide from
tRNA at P site
Understand that eukaryotes commonly have "intervening sequences" (Introns) in their
genes and in the initial RNA that is transcribed. Introns must be removed to produce a
mature mRNA that can be translated.
Understand that the expression of many genes is regulated. Genes are expressed at
appropriate times.
Know that the expression of genes can be regulated by influencing the rate at which
genes are transcribed or the rate at which they are translated.
Most gene regulation is at the level of transcription.
Transcription can be enhanced or block by regulatory proteins that interact with the DNA.
o Repressor proteins bind to the DNA and block transcription - this is negative
o Activator proteins bind to the DNA and allow transcription - this is positive
In prokaryotes genes that have related functions are grouped together and regulated
together. These groups and they regulatory regions are called an operon.
The lac operon is a group of three genes that promote the metabolism of the
disaccharide lactose.
The lac operon has both positive and negative regulation
o a repressor protein binds to the DNA in the operator region and blocks
transcription when lactose is not present. When lactose is present it binds to the
repressor and the repressor leaves the DNA, potentially allowing transcription
o an activator protein (the catabolite activator protein, CAP) must be bound to the
DNA in the promoter region for transcription to occur. It will only be bound to
DNA when energy levels in the cell are low
energy levels in the cell will be low when the cell has no glucose
available to it
 when energy levels are low the cell produces cyclic AMP (cAMP)
 when cAMP is present in the cell, it associates with CAP to form the
CAP-cAMP complex. This complex can bind to the DNA to promote
o for the lac operon genes to be expressed lactose must be present (removes the
repressor) and energy levels in the cell must be low (CAP-cAMP must be bound)
The trp operon regulates the expression of genes that code for the enzymes that
synthesize tryptophan
o a repressor protein binds to the DNA when tryptophan is present in the cell and
blocks transcription
o when tryptophan is absent the repressor releases the DNA and allows
o after transcription, tryptophan levels rise and block transcription until tryptophan
is depleted
Eukaryotic genes are not usually grouped in operons - they are regulated independently
Eukaryotic gene regulation is more complex in that commonly many more regulatory
proteins control gene expression (repressors, activators, co-activators, enhancers, basal
factors, etc.). Each regulatory protein is produced under specific conditions. So, many
conditions must be satisfied simultaneously for transcription of many eukaryotic genes.
Eukaryotic genes found within tightly coiled regions of the DNA (heterochromatin) are not
Regulation of gene expression after transcription can occur at many levels
o introns must be removed for normal expression
o a sequence of adenines (a poly-A tail) must be added to the 3' end of the mRNA
before it can leave the nucleus
o proteins also regulate the rate of translation in the cytoplasm
o some mRNAs are degraded more quickly than others in the cytoplasm
Understand that different alleles of a gene are most commonly the result of small
differences in the sequence of nucleotides
A small change in the nucleotide sequence of a genes is called a point mutation
Point mutations can be classified based on their effects on the amino acid sequence in
the protein that the gene codes for
o a missense mutation is a base pair change that results in one amino acid being
substituted for another
o a nonsense mutation is a base pair change that results in an amino acid coding
codon being replaced by a stop codon (eg. UGG becomes UGA)
o a silent mutation is a base pair change that results in no amino acid change
because of redunancy in the genetic code
o a frameshift mutation is the result of the addition or deletion of bases from the
DNA. Frameshift mutations can result in a change in many amino acids in the
Mutation of any given gene is rare. About one in every 100,000 gametes carries new an
allele for specific gene that resulted from a new mutation
The human genome contains approximately 40,000 genes. About 4 in every 10 gametes
carry a new allele for one of the genes.
Most mutations are deleterious, but some are beneficial and are the raw material for new
Many mutational events are short-lived because of DNA repair enzymes. Only those that
do not get repaired are be passed to the next generation or to daughter cells.
Many things cause mutations (chemicals, radiation, heat, ...)
There is a strong link between mutations and cancer. Mutations in cells within the body
can result in inappropriate gene expression and uncontrolled cell growth (cancer).
Understand the human male and female reproductive anatomy.
Understand the role that hormones have in regulating human gametogenesis
Understand the difference between the ovarian and uterine cycles
Understand the basic development patterns for mammals, insects, and nematodes
Understand the basic mechanisms for animal development
Know what homeotic genes are and what they do
Know what induction is
Know the relationship between organizers and morphogens
Understand determination and differentiation in relation to animal development
Know what totipotent means