Genetics: Mutation Types and DNA Repair

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Questions and Answers

What is catabolite repression primarily responsible for in bacterial cells?

Promoting the accumulation of cyclic AMP

Inhibiting the use of carbon sources other than glucose (correct)

Encouraging the use of all available carbon sources

Facilitating rapid growth on lactose

What happens to cyclic AMP (cAMP) levels in a cell when glucose is not available?

cAMP levels build up (correct)

cAMP levels remain unchanged

cAMP levels decrease significantly

cAMP is rapidly broken down

What role does cyclic AMP (cAMP) play in the context of lactose utilization?

It inhibits lactose utilization

It activates the promoter for glucose metabolism

It binds to catabolic activator protein (CAP), enhancing transcription (correct)

It directly metabolizes lactose

How does the growth rate of E. coli compare when using glucose versus lactose as the only carbon source?

E. coli grows faster on glucose than on lactose (B) Signup and view all the answers

What occurs during the lag time when E. coli switches from glucose to lactose consumption?

Glucose is consumed first before lactose uptake begins (B) Signup and view all the answers

What is the primary function of the repressor protein in the operon model?

It blocks transcription of genes. (B) Signup and view all the answers

What happens to the operon when the inducer allolactose is present?

Transcription of structural genes occurs. (B) Signup and view all the answers

What is the default position of an inducible gene in the absence of an inducer?

It is off and not transcribed. (A) Signup and view all the answers

Which of the following statements about repressible genes is correct?

They can be switched off by repressors. (B) Signup and view all the answers

Which component prevents transcription in the absence of lactose in the LAC operon?

The repressor protein. (A) Signup and view all the answers

What triggers transcription of structural genes in an inducible operon?

Presence of the inducer. (D) Signup and view all the answers

What is the role of RNA polymerase in the operon model?

To facilitate transcription of genes. (D) Signup and view all the answers

Which enzyme is produced as a result of gene expression from the LAC operon?

Beta-galactosidase. (C) Signup and view all the answers

What is a mutation?

A change in the sequence of bases in an organism’s DNA. (D) Signup and view all the answers

Which type of mutation results in a nonsense codon?

Nonsense mutation (C) Signup and view all the answers

What is the first step in the DNA repair process?

Recognition (A) Signup and view all the answers

What causes spontaneous mutations?

Absence of a mutagen (D) Signup and view all the answers

Which of the following is an example of a chemical mutagen?

Nitrous acid (D) Signup and view all the answers

What is the effect of a frameshift mutation?

It shifts the translational reading frame. (B) Signup and view all the answers

What can result from the failure to repair DNA damage?

Development of serious diseases (D) Signup and view all the answers

What is the primary consequence of UV radiation on DNA?

It causes thymine dimers. (B) Signup and view all the answers

Which mutation leads to a change in the amino acid sequence?

Missense mutation (B) Signup and view all the answers

What role do specialized enzymes play in DNA repair?

They identify, remove, and replace damaged DNA. (B) Signup and view all the answers

What triggers the activation of CAP in the lac operon?

High levels of cAMP (A) Signup and view all the answers

What is the effect of low cAMP levels on the lac operon?

Decreased transcription of the lac genes (D) Signup and view all the answers

In the presence of lactose and glucose, what happens to the lac operon?

The lac operon remains inactive due to an active repressor. (D) Signup and view all the answers

Under which condition does the lac operon produce large amounts of mRNA for lactose digestion?

Lactose present and glucose scarce (D) Signup and view all the answers

Which molecule cannot bind to the lac operon when glucose levels are high?

CAP (C) Signup and view all the answers

What role does the inactive lac repressor play when lactose is present?

It inhibits transcription of the lac operon. (D) Signup and view all the answers

What happens to CAP when cAMP levels are low?

It becomes inactive. (D) Signup and view all the answers

What is the relationship between lactose and the lac operon in the transcription process?

Lactose binds to the lac repressor, inhibiting its function. (B) Signup and view all the answers

What role does tryptophan play in a repressible operon?

It functions as a corepressor. (C) Signup and view all the answers

In the absence of glucose and the presence of lactose, what is the level of transcription?

Strong transcription occurs. (C) Signup and view all the answers

Which of the following components is NOT part of the operon structure?

Repressor protein (B) Signup and view all the answers

What occurs when excess tryptophan is present in a repressible operon?

The repressor binds to the operator. (B) Signup and view all the answers

What is the primary function of the regulatory gene in an operon?

To produce the repressor protein. (D) Signup and view all the answers

Under what conditions would low-level transcription of the lac operon occur?

High glucose and no lactose. (C) Signup and view all the answers

Which site in the operon is directly involved in controlling the binding of RNA polymerase?

Promoter site (A) Signup and view all the answers

What happens to mRNA synthesis when the repressor is inactive?

mRNA synthesis occurs normally. (C) Signup and view all the answers

What occurs when the corepressor tryptophan binds to the repressor protein?

The activated repressor binds to the operator. (D) Signup and view all the answers

How do microRNAs (miRNAs) influence gene expression?

They degrade the mRNA of target genes. (A) Signup and view all the answers

What is the effect of methylating nucleotides on gene expression?

It turns genes off. (D) Signup and view all the answers

Which statement accurately describes a riboswitch?

It changes mRNA structure upon substrate binding. (B) Signup and view all the answers

What role does epigenetic control play in gene expression?

It modifies gene expression via chromosome packing. (C) Signup and view all the answers

What happens to tightly packed DNA in relation to transcription?

It prevents access to transcription machinery. (D) Signup and view all the answers

How does a cell regulate the lifespan of mRNA?

By utilizing enzymes that quickly degrade mRNA. (C) Signup and view all the answers

What is the main function of post-transcriptional control?

To regulate mRNA processing and stability. (D) Signup and view all the answers

Study Notes

Mutation

A mutation is a permanent change in the base sequence of DNA.
Mutations can be neutral, beneficial, or harmful.
Mutagens are substances or energy sources that induce mutations.
Spontaneous mutations occur in the absence of mutagens.
Mutations may involve a single base change or large-scale chromosomal abnormalities.
New alleles arise due to mutations.
Most genetic mutations are neutral or harmful.

Types of Mutations

Base substitution (point mutation): A change in one base in DNA.
- Missense mutation: A base substitution that results in a change in the corresponding amino acid.
- Nonsense mutation: A base substitution that results in a nonsense (stop) codon.
- Silent mutation: A base substitution that does not result in a change in the amino acid.
Frameshift mutation: An insertion or deletion of one or more nucleotide pairs, causing a shift in the reading frame during translation.

DNA Repair

DNA repair is a three-step process:
- Recognition: Repair proteins detect and tag damaged DNA.
- Removal: Repair enzymes remove the damaged segment.
- Replacement: A repair DNA polymerase fills the gap with the correct sequence of bases.
Each step employs different repair proteins and specialized enzymes.
Damaged DNA is removed and replaced.

DNA Damage and Diseases

Failure to repair DNA damage can lead to serious diseases like cancer.
UV radiation can cause thymine dimers.
Ionizing radiation, such as X-rays and gamma rays, causes the formation of ions that can oxidize nucleotides and break the deoxyribose-phosphate backbone.
Xeroderma pigmentosum (XP) is a condition where people lack the functional versions of repair proteins, and they accumulate mutations leading to skin cancer.

Mutations Affecting Protein Function

Even single-base changes can alter protein function enough to produce a harmful phenotype (e.g., a disease).
Frameshift mutations alter protein structure extensively, typically destroying its normal function and causing severe phenotypes.
Silent mutations have no impact on the protein structure or phenotype.
Rare mutations can be beneficial, improving protein efficiency or functionality.

Operon Model of Gene Expression

Promoter: A DNA segment where RNA polymerase initiates transcription.
Operator: A DNA segment that controls the transcription of structural genes.
Operon: A set of operator and promoter sites and the structural genes.
Inducible Operon: Structural genes are not transcribed unless an inducer is present.
Repressible Operon: Structural genes are transcribed until they are turned off.

lac Operon

Regulation of the lac operon:
- In the absence of lactose, a repressor binds to the operator, preventing transcription.
- In the presence of lactose, allolactose (an inducer) binds to the repressor, preventing it from binding to the operator, allowing transcription to occur.

lac Operon- Positive Regulation

Catabolite repression: Cells preferentially use glucose; cAMP builds in the absence of glucose.
In the absence of glucose, cAMP binds to CAP (catabolic activator protein), which then binds to the promoter of the lac operon.
Activating lac transcription.

Repressible Operon (Tryptophan Operon)

Structural genes are transcribed until excess tryptophan is available.
Tryptophan acts as a corepressor by binding to the repressor protein and activating it, allowing it to bind to the operator and prevent further transcription.

Post-transcriptional Control

Riboswitches: mRNA molecules that bind to substrates to change their structure.
microRNAs (miRNAs): Base pair with mRNA, causing it to become double-stranded and leading to degradation. Preventing protein synthesis.
Epigenetics: Methylation of nucleotides turn genes off, and methylation patterns may be inherited.

Gene Expression Regulation

Tight packing of DNA makes genes transcriptionally inactive.
Regulation of transcription conserves cellular resources.
mRNA is degraded to avoid protein synthesis.
Translation is regulated to keep mRNA ready.
Proteins are directly regulated after synthesis.

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Description

This quiz explores the various types of mutations, including base substitutions and frameshift mutations, as well as the mechanisms of DNA repair. Understand how mutations can alter genetic information and their implications in biology. Test your knowledge on neutral, beneficial, and harmful mutations.