Quiz #11 Flashcards
What are mutations?
heritable changes in DNA sequence, as compared to a “normal” reference sequence, the reference is usually referred to as wild type
a change in DNA might nor change any characteristics or functions of a cell (or organism), it depends on the location (in the protein) and nature of the mutation
What are the structures a mutation can effect?
proteins: by altering the coded information in different ways
rRNAs, tRNAs, or other non-coding RNA (e.g. snRNP RNA)
regions of DNA that have other functions that depend on the sequence (e.g. promoters, origins, centrosomes)
What is an allele?
each variant form of a gene
a diploid organism has two alleles of each gene (may be the same or different, AA or Aa or aa)
the most common allele in a population is often referred to as “wild type”
What is a substitution mutations?
base-pair substitutions are the replacement of one nucleotide and it’s partner with another pair
What are the different types of substitution mutations?
silent mutations
missense mutations
nonsense mutations
What are silent mutations?
because of redundancy in the genetic code, some mutations do not change the primary sequence of a polypeptide encoded by a gene
always has no effect on the amino acid sequence, but the efficiency of translation might change due to the different codon used
What are missense mutations?
result in a change in the primary sequence of a polypeptide
may have little or no effect on a protein’s function
if it changes an amino acid within the active site of an enzyme it could cause it not to work, but if it changes an amino acid on the outside of an enzyme it might not affect function
What are nonsense mutations?
changes a codon for an amino acid into a STOP codon
causes premature termination of translation
most nonsense mutations lead to non-functional proteins
a large portion of protein could be missing if it occurs early
What are insertion or deletion mutations?
all nucleotides downstream of a frameshift mutation will be improperly grouped into codons that convey extensive missense information
an insertion or deletion of 3 nucleotides would result in one less or one more amino acid but no reading frame shift
eventually, a STOP codon will be encountered, or the ribosome will reach the end of the mRNA
What is an example of a mutation that causes a disease?
the common type form of sickle cell anemia is caused by a single base pair substitution in the gene that encodes the beta hemoglobin polypeptide
What are spontaneous mutations?
most due to errors in DNA replication that are not corrected
What are induced mutations?
caused by mutagens, x-rays, gamma rays, nucleotide analogs, oxidizing agents, benzene
What are carcinogens?
mutagens that are known to cause cancer
How does the regulation or control of genes happen?
by turning gene transcription on or off, a cell controls which proteins it makes under different conditions
in prokaryotes (bacteria) or simple eukaryotes (yeasts), gene expression is often directly influenced by the environment (which nutrients are present or absent)
What are the two general ways genes may be transcribed in prokaryotes?
single units (one gene transcribed from one promoter)
multiple gene units that share the same promoter (derived from genes that occur in clusters called operons)
What is tryptophan (Trp)?
is an amino acid required for growth, and is present in virtually all proteins in a cell
if Trp accumulates, it can bind to Enzyme 1 (allosteric inhibition) and shut it off
the cell can quickly respond to fluctuating levels of Trp
What are the steps of regulation of tryptophan synthesis in bacteria?
- Trp synthesis is regulated at the protein level by allosteric inhibition
- Trp synthesis is controlled via transcriptional regulation of the Trp operon
- Trp synthesis is also regulated by attenuation, which interferes with the completion of transcription
How is Trp synthesis controlled via transcriptional regulation of the Trp operon?
mechanism was discovered in E. coli bacteria
involves a cluster of genes that encode enzymes required for the synthesis of Trp
operon is expressed (on) when Trp levels are low and repressed (off) when Trp levels are high
operon is regulated by the Trp repressor, encoded by the TrpR gene, when bound to Trp, the Trp repressor binds to the Trp operator sequence and blocks transcription of Trp genes
How is Trp synthesis regulated by attenuation which interferes with the completion of transcription?
woks because of coupled transcription and translation in prokaryotes
monitors the availability of Trp amino acids for incorporation into polypeptides
What does gene repression look like in prokaryotes?
initiation of transcription is relatively simple (sigma and a promoter is sufficient to transcribe most typical genes)
thus repression is often used to prevent transcription of some genes when the proteins they encode are not needed (Trp operon)
What are inducible operons?
those that are OFF most of the time, but can be induced or turned ON
What are some examples of inducible operons in prokaryotes?
Repressor is always bound to the operator except under certain conditions
When the promoter is imperfect and requires another protein (that is regulatable) to help recruit the RNA polymerase
What does gene repression look like in eukaryotes?
in many cases, another region of DNA “loops around” and is used to help recruit the RNA pol II (these are referred to as enhancer elements)
differential gene expression determines a cell’s characteristics and this is how complex organisms create specialized cells and/or tissues with the same DNA
the decision of whether to transcribe a gene or not, starts with the compaction state of the chromatin where the gene is located
What is the difference between heterochromatin and euchromatin in eukaryotes?
DNA in eukaryotes can exhibit multiple levels of compaction, due to nucleosomes and higher-order structures
heterochromatin: highly condensed
euchromatin: not condensed
because of it’s compaction, heterochromatin is largely inaccessible to transcription factors, loosely packed euchromatin is more accessible to the transcription factors
