Ch. 11

What are the two essential functions of DNA?

To store genetic information and to direct the synthesis of proteins.

What is the central dogma of genetics?

DNA is transcribed into RNA, which is then translated into proteins.

What is meant by the term gene expression?

The process by which information from a gene is used to produce a functional gene product, typically a protein.

Are all genes in a cell expressed at all times?

No, genes are regulated and only expressed as needed.

What is the difference between a constitutive gene and a regulated gene?

Constitutive genes are continuously expressed while regulated genes are expressed only under certain conditions.

Explain the difference between genotype and phenotype.

Genotype refers to the genetic constitution of an organism, while phenotype is the observable physical and functional traits.

Is phenotype solely a result of genotype? Explain.

No, phenotype is influenced by both genotype and environmental factors.

Why is DNA replication described as semi-conservative?

Because each new DNA molecule consists of one original strand and one newly synthesized strand.

What is an origin of replication?

The specific location where DNA replication begins.

Distinguish between prokaryotes and eukaryotes in terms of how many origins of replication the chromosomes have.

Prokaryotes typically have a single origin of replication, whereas eukaryotes have multiple origins per chromosome.

Why are topoisomerases needed for replication?

Topoisomerases help relieve the tension and supercoiling that occurs ahead of the replication fork.

Describe the function of helicase in DNA replication.

Helicase unwinds the DNA double helix at the replication fork.

Describe the function of primase in DNA replication.

Primase synthesizes RNA primers to initiate DNA synthesis.

Describe the function of DNA polymerase in DNA replication.

DNA polymerase synthesizes new DNA strands by adding nucleotides complementary to the template strand.

Describe the function of ligase in DNA replication.

Ligase connects Okazaki fragments on the lagging strand by forming phosphodiester bonds.

In what direction does replication occur? (5’ to 3’ or 3’ to 5’?)

Replication occurs in the 5' to 3' direction.

During replication, why is one strand called the 'leading strand' and the other the 'lagging strand'?

The leading strand is synthesized continuously, while the lagging strand is synthesized in segments called Okazaki fragments.

What are Okazaki fragments?

Short sequences of DNA synthesized on the lagging strand during replication.

What enzyme joins Okazaki fragments together?

Ligase.

What do we mean when we say DNA polymerases have 'proofreading ability'?

DNA polymerases can identify and correct errors in newly synthesized DNA.

Why is it important for DNA polymerase to 'proofread' its work?

It helps maintain the integrity of the genetic information by reducing mutations.

Topoisomerase IV is a type of topoisomerase specifically found in prokaryotes. Why do these cells need this enzyme?

To resolve tangles and supercoils that occur during DNA replication.

What is the purpose of transcription?

To synthesize RNA from a DNA template.

What role does the promoter serve during transcription?

The promoter is a DNA sequence that signals the start of transcription.

Where is the promoter found in relation to the gene?

The promoter is located upstream of the gene it regulates.

Which enzyme is the key enzyme needed to produce mRNA?

RNA polymerase.

Where does transcription occur in eukaryotic cells?

In the nucleus.

Where does transcription occur in prokaryotic cells?

In the cytoplasm.

In what direction does transcription occur? (5’ to 3’ or 3’ to 5’?)

Transcription occurs in the 5' to 3' direction.

During transcription, why is one strand of DNA considered the 'coding' (or 'sense') strand, and the other the 'template' (or 'antisense') strand?

The coding strand has the same sequence as the mRNA, while the template strand is complementary to the mRNA.

During transcription, which nucleotide pairs with adenine (A)?

Uracil (U).

How does RNA polymerase 'know' when it has reached the end of a gene?

It recognizes a specific termination sequence in the DNA.

What is the purpose of translation?

To synthesize proteins from mRNA.

What two types of RNA are needed for translation, in addition to mRNA?

Transfer RNA (tRNA) and ribosomal RNA (rRNA).

Describe the role of tRNA in translation.

tRNA brings amino acids to the ribosome and matches them with the codons on mRNA.

Describe the role of rRNA in translation.

rRNA makes up the ribosome and helps facilitate the process of translation.

What is a codon?

A sequence of three nucleotides on mRNA that codes for a specific amino acid.

How many different codons are there?

There are 64 different codons.

How many of these are sense codons?

61 sense codons.

How many of these are nonsense (stop) codons?

3 nonsense codons.

How many of these are start codons?

1 start codon (AUG).

What do we mean when we say the genetic code is universal?

It is the same across almost all organisms.

What do we mean when we say the genetic code is triplet?

Each codon comprises three nucleotides.

What do we mean when we say the genetic code is overlapping?

Each nucleotide can be part of more than one codon when the reading frame shifts.

Describe some ways in which transcription and translation in prokaryotes differs from eukaryotes.

Prokaryotes transcribe and translate simultaneously, while eukaryotes have separate processes in the nucleus and cytoplasm.

What is a mutation?

A change in the DNA sequence.

Are mutations always bad? Explain.

No, mutations can be neutral, beneficial, or harmful depending on their effects.

What is the difference between a spontaneous mutation and an induced mutation?

Spontaneous mutations occur naturally, while induced mutations result from exposure to mutagens.

What is a mutagen?

A substance that causes mutations.

Why are many mutagens also called carcinogens?

Because they can lead to cancer by causing changes in DNA.

What is a point mutation?

A mutation that alters a single nucleotide in the DNA sequence.

What is an insertion?

A mutation involving the addition of one or more nucleotide pairs to the DNA sequence.

What is a deletion?

A mutation involving the loss of one or more nucleotide pairs from the DNA sequence.

What happens with a missense mutation?

It results in a single amino acid change in the protein.

What happens with a silent mutation?

It does not change the amino acid sequence of the protein.

What happens with a nonsense mutation?

It creates a premature stop codon, truncating the protein.

What happens with a frameshift mutation?

It shifts the reading frame, causing changes to all downstream amino acids.

What is horizontal gene transfer?

The transfer of genetic material between organisms other than by descent.

Why is horizontal gene transfer especially important in prokaryotes?

It allows for rapid genetic diversification and adaptability.

For transformation, where is the foreign DNA coming from?

From the environment.

For conjugation, where is the foreign DNA coming from?

From another bacterium through direct contact.

What cellular appendage is needed for conjugation?

Pilus.

For transduction, where is the foreign DNA coming from?

From a bacteriophage (virus) that infects bacteria.

What acts as the 'delivery vehicle' during transduction?

Bacteriophage.

What is a transposon?

A sequence of DNA that can move to different positions within the genome.

Why are transposons also called 'jumping genes'?

Because they can relocate within the genome.

Which scientist discovered transposons?

Barbara McClintock.

What are two possible effects a transposon could have on a cell?

It can disrupt genes or regulatory elements, or promote genetic diversity.

What is an operon?

A group of genes regulated together, typically involved in a related function.

Do all cells have operons?

No, operons are primarily found in prokaryotes.

In an operon, what is the function of an operator?

It serves as a regulatory element that controls the transcription of the operon.

Where is the operator found?

Within the promoter region of the operon.

In the lac operon, what role does the repressor serve?

The repressor inhibits transcription in the absence of lactose.

What will cause the repressor to be released from the DNA?

The presence of lactose.

What will happen to the operon when the amount of lactose in the cell goes back down?

The repressor will bind to the operator, preventing transcription.