EXAM 3 Stuff
A polycistronic mRNA is a single RNA molecule that contains the coding sequences for multiple proteins (typical in prokaryotes like bacteria).
Divided into several regions called cistrons
Each cistron has its own ribosome binding site (RBS) in prokaryotes, allowing for translation initiation at different points within the polycistronic transcript.
Ex. Lac operon in bacteria
Polypeptide-Encoding Sequences are sequences of nucleotides in the mRNA that encode the information for the synthesis of a polypeptide (a chain of amino acids).
Make up the functional part of polycistronic mRNA
During translation, ribosomes read these polypeptide-encoding sequences in sets of three nucleotides (codons), each specifying an amino acid.
Elongation factors are essential proteins involved in the process of translation elongation during protein synthesis. They facilitate key steps that allow the ribosome to move along the mRNA and synthesize a protein.
Prokaryotic Elongation factors: EF-Tu (delivers aminoacyl-tRNA to the ribosome's A-site) and EF-G (facilitates the translocation of the tRNA and mRNA complex on the ribosome after a peptide bond is formed, essentially moving the ribosome one codon forward to read the next amino acid.)
Eukaryotic Elongation factors: eEF1 (delivers amino acids to the ribosome) and eEF2 (moves the ribosome along the mRNA)
Translocation Steps:
The tRNA enters the A site with the help of elongation factors (EF-Tu or eEF1).
A peptide bond is formed.
EF-G (prokaryotes) or eEF2 (eukaryotes) hydrolyze GTP to drive the ribosome to translocate along the mRNA by one codon.
The ribosome shifts, and the process repeats.
The tRNA (transfer RNA) recognizes mRNA codons through its anticodon region.
For example, the anticodon UAC (on tRNA) is complementary to the codon AUG (on mRNA).
The anticodon and the codon bind through Watson-Crick base pairing:
A (adenine) pairs with U (uracil in RNA) or T (thymine in DNA).
U (uracil) pairs with A (adenine).
C (cytosine) pairs with G (guanine).
G (guanine) pairs with C (cytosine).
Insertions in coding sequences can lead to frameshift mutations (changing the reading frame) or the addition of extra amino acids, both of which can significantly alter protein function.
Insertions in non-coding regions can affect gene regulation, mRNA processing, and protein expression, leading to altered gene activity or even disease.
Mutation in tRNA anticodons can have a variety of effects, depending on the type of mutation and its location.
Missense mutations lead to incorrect amino acid incorporation.
Nonsense mutations lead to premature termination of translation.
Silent mutations may not have any noticeable effects.
Suppressor mutations can sometimes compensate for the initial mutation.
Types of Nucleotide Codon Mutations:
Substitutions (Single Nucleotide Changes) i.e silent, missense, nonsense
Insertions (Adding Nucleotides) ——————\ Frameshift
Deletions (Removing Nucleotides) —————/ Frameshift
A paracentric inversion occurs when a chromosomal segment is reversed within the same arm of a chromosome, not involving the centromere.
A pericentric inversion involves a chromosomal segment that is reversed, and the inversion includes the centromere.
Both can result in an inversion loop!
An inversion loop forms during meiosis when homologous chromosomes with inversions try to pair up and align for recombination.
Unpaired loops occur when there is no inversion but rather a structural mismatch between homologous chromosomes during meiosis. (Usually by insertion or deletion)
A dicentric bridge is a structure that forms during meiosis or mitosis when dicentric chromosomes (chromosomes with two centromeres) undergo abnormal behavior during chromosome segregation. Can break chromosome or cause unequal segregation!
A transposon is a segment of DNA that moves, while transposase is the enzyme that cuts and pastes the transposon.
An unbalanced translocation occurs when there is an exchange of chromosomal material between two chromosomes, but the transfer of material is unequal.
A Robertsonian translocation is a specific type of chromosomal rearrangement that occurs between two acrocentric chromosomes (chromosomes with very short p arms and long q arms), where the two long arms of the chromosomes fuse together, and the short arms are typically lost. (Accrocentric only!)
The Shine-Dalgrano sequence in bacteria is a purine-rich consensus sequence found in the 5’ UTR of the mRNA.
During translation initiation in bacterial, the amino acid on the initiator tRNA is N-formylmethionine (fMet)
How does the eukaryotic initiation complex locate the correct start code on? The initiation complex moves the small ribosomal subunit through the 5’ UTR, scanning for the start AUG.
Locations of stages of translation in bacteria and eukaryotes: cytosol in bacteria; and cytosol, mitochondrion, and plastid for eukaryotes.
A tRNA in the AP site of the ribosome will enter the E site after translocation of the ribosome
A release factor is never found in the A site of the ribosome.
Two-dimensional gel electrophoresis separates proteins based on two key features: their isoelectric point (pI), which determines their charge at a specific pH, and their molecular weight (size).
If non-disjunction occurs in meiosis one of a mother, half of her traits could have been fertilized by a normal sperm to produce a child with Turner syndrome.
An acentric chromosome fragment is produced by recombination between a rearranged and normal homolog in Paracentric inversion.
Prader-Willi and Angelman syndromes are caused by which type of chromosomal mutations, both in connection with chromosome 15? Uniparental disomy
What technique would you use to detect a target sequence in an intact chromosome using a labeled molecular probe?
A fluctuation test is a method used to measure mutation rates in microorganisms and to determine if mutations occur randomly.
Fluorescent in situ hybridization (FISH)
The "phr" gene in E. coli encodes the enzyme photolyase, which is responsible for repairing DNA damage caused by ultraviolet (UV) light through a process called photoreactivation
In an allopolyploid organism, it is true that the fertility of interspecies hybrids is high.
Turner syndrome is caused by a chromosomal deletion or mutation, resulting in the absence or partial absence of an X chromosome.
lacZ: encodes beta-galactosidase, which breaks down lactose into glucose and galactose.
lacY: encodes lactose permease, a protein that transports lactose into the cell
lacA: encodes beta-galactoside transacetylase, an enzyme with a less critical role in lactose metabolism
When lactose is absent, the lac repressor (produced by the lacI gene) binds to the operator and prevents transcription of the operon, meaning the genes for lactose utilization aren’t expressed.
When lactose is present, lactose molecules (or allolactose, a derivative) act as an inducer. They bind to the lac repressor, causing a conformational change that releases the repressor from the operator region. This allows RNA polymerase to bind to the promoter and initiate transcription of the lac genes, enabling the bacteria to produce the enzymes necessary for lactose metabolism.
The lac operon allows bacteria to conserve energy by only producing the enzymes required to metabolize lactose when it is present.
What is meant by the "beads on a string" model of chromatin? The beads are the histones, and the string is the transcriptionally active DNA loops.
The trp operon, found in E. coli bacteria, is a group of genes that encode biosynthetic enzymes for the amino acid tryptophan.
What did Hermann Muller find out about Drosophila? that exposing fruit flies (Drosophila) to X-rays significantly increased the rate of genetic mutations in their offspring, demonstrating that radiation could induce mutations in genes
Mutation rate: measured by counting the number of mutations affecting a phenotype and determining that frequency of mutations per base pair
Mutations are rare but genomes are genuinely considered to be stable!
Point mutations: occur at a specific position in a gene - in a gene (alter amino acid sequence) - in a regulatory region (affects gene expression levels)
Silent mutations are due to the reduction in the effectiveness of a codon to specify an amino acid, resulting in no change to the protein produced, thus maintaining the overall function of the gene.
Synonymous mutation: a type of silent mutation where a change in the nucleotide sequence does not alter the amino acid sequence of the protein, thereby preserving its function and stability.
Missense mutation: a type of point mutation where a single nucleotide change results in the substitution of one amino acid for another in the protein sequence, which can potentially alter the protein's function or stability.
Nonsense mutation: a type of point mutation that results in a premature stop codon in the amino acid sequence, leading to the truncation of the protein and often resulting in a loss of function.
Frameshift mutation: a type of mutation caused by insertions or deletions of nucleotides in the DNA sequence that shifts the reading frame of the codons, often resulting in a completely different and nonfunctional protein.
Triplet repeat expansion: a type of mutation characterized by the increase in the number of times a specific DNA sequence, typically consisting of three nucleotides, is repeated, which can lead to genetic disorders by disrupting normal gene function.
Depurination: the loss of a purine base (adenine or guanine) from the DNA molecule, which can result in mutations during DNA replication if not repaired properly.
Apurinic site: a site in DNA that has lost a purine base due to depurination, which can lead to replication errors and mutations if the missing base is not replaced.
Deamination: the removal of an amino group from a nucleotide, which can convert cytosine into uracil, causing base-pairing issues during DNA replication
Alkylating agents can result in base substitution mutations
Photoreactive repair: a DNA repair mechanism that uses visible light to repair damage caused by ultraviolet (UV) light - repairs thymine dimers
Nucleotide Excision Repair: a DNA repair mechanism that removes damage from DNA (also known as ultraviolet repair)
Nearly half of the human genome is composed of transposable DNA (47 or 48%)
A core nucleosome is made up of a protein complex called a histone octamer, consisting of two copies each of the four core histone proteins (H2A, H2B, H3, and H4), with approximately 146 base pairs of DNA wrapped around it, forming a spool-like structure
What would be the phenotype of an int λ mutant? The phage would be able to enter the lytic cycle, but would not be able to lysogenize.