The Central Dogma of Biology

Who discovered the phenomenon of bacterial transformation in Streptococcus pneumoniae?

Frederick Griffith

What type of virus did Hershey and Chase use to prove that DNA is the genetic material?

Bacteriophage (specifically T2 phage)

Which three scientists are credited for discovering the structure of DNA?

Rosalind Franklin, James Watson, and Francis Crick

What is the central dogma of biology?

DNA → RNA → Protein

What technique did Rosalind Franklin use to capture the structure of DNA?

X-ray crystallography (X-ray diffraction)

How did Griffith’s experiment support the idea that DNA carries genetic information?

He showed that a "transforming factor" from dead pathogenic bacteria could genetically change harmless bacteria, implying the inheritance of traits.

Why was the Hershey-Chase experiment more convincing than Griffith’s?

It used radioactive labeling to directly show that only DNA, not protein, entered bacteria and directed the production of new viruses.

What structural features of DNA allow it to carry genetic information effectively?

Its double-helix structure, complementary base pairing (A-T, C-G), and ability to replicate allow it to store and transmit genetic information.

What is the role of the sugar-phosphate backbone in DNA?

It forms the structural framework of the DNA strand, keeping the molecule stable and directing base pairing inward.

A researcher destroys proteins in a bacterial sample, yet the sample still passes on traits to other bacteria. What does this suggest about the genetic material?

The genetic material is not protein, but likely DNA, since traits were still inherited without proteins.

In an experiment, radioactive sulfur is found outside the infected bacterial cells while radioactive phosphorus is found inside. What conclusion can be drawn?

DNA (containing phosphorus) enters the host cell and is the genetic material, while protein (containing sulfur) remains outside.

If a mutation occurs in DNA but the resulting protein is unchanged, which part of the central dogma buffered this change?

The redundancy in the genetic code during translation (RNA to protein) buffered the mutation.

In the lab, a student observes viral DNA being injected into bacteria and later sees the bacteria burst, releasing new viruses. What viral process is being observed?

The lytic cycle of a bacteriophage infection.

Genotype

is an organism’s genetic makeup and is the heritable information stored in the nucleotide sequences of DNA.

Phenotype

is the organism’s physical traits.

“central dogma of biology”

which refers to the flow of genetic information and its expression. This dogma tells us that the molecular chain of command is from the DNA in the cell, to RNA, to protein synthesis in the cytoplasm. The two main stages are transcription, the synthesis of RNA based on instructions from DNA; and translation, the synthesis of protein under the direction of RNA.

Transcription in Prokaryotes

The transcription process in prokaryotes is simpler compared to eukaryotes. Once the DNA strands are separated, one strand serves as a template for a new RNA molecule, and the other is unused.

The enzyme for transcription is RNA polymerase which moves along the gene, following the base-pairing rules. A specific sequence called promoter acts as a binding site for RNA polymerase and determines where transcription starts. RNA polymerase adds to the chain until it reaches a sequence of DNA bases called the terminator, which signals the end of the gene.

The transcription of the gene in prokaryotes is composed of three major steps:

1. Initiation – which involves the attachment of RNA polymerase to the promoter to start RNA synthesis.

2. Elongation – the newly formed RNA strand grows with the growing RNA peeling away from its DNA template. This allows the separated DNA strands to come back together in the region already transcribed

3. Termination – When RNA polymerase reaches the terminator DNA, the polymerase molecule detaches from the newly formed RNA strand and the gene.

What molecule serves as the bridge between DNA and protein synthesis?

RNA (ribonucleic acid).

What is transcription?

The process of synthesizing RNA from a DNA template.

What is translation?

The process of synthesizing a protein from an mRNA template.

What is a codon?

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

What is the start codon and what does it code for?

AUG; it codes for methionine (Met) and signals the start of translation.

What are the stop codons?

UAA, UGA, and UAG.

What are introns and exons?

Introns are non-coding regions of RNA removed during RNA splicing; exons are the coding regions joined together.

What is the role of mRNA?

To carry genetic instructions from DNA to the ribosome for protein synthesis.

What does tRNA do?

It brings amino acids to the ribosome and matches them with mRNA codons via its anticodon.

What is the function of rRNA?

It forms part of the ribosome and helps catalyze protein synthesis.

Why is the genetic code considered a triplet code?

Because each amino acid is encoded by a set of three nucleotides (codon), allowing for 64 combinations to code 20 amino acids.

How is RNA structurally different from DNA?

RNA uses ribose sugar instead of deoxyribose, is single-stranded, and has uracil (U) instead of thymine (T).

Why must eukaryotic mRNA be processed before translation?

To add protective caps/tails, remove introns, and ensure accurate translation of coding sequences.

What ensures that the correct amino acid is matched with the correct codon during translation?

The anticodon on tRNA base-pairs with the codon on mRNA, and specific enzymes attach the right amino acid to each tRNA.

What happens at the P and A sites of a ribosome during translation?

The P site holds the growing polypeptide; the A site holds the incoming aminoacyl-tRNA.

A mutation removes the 5’ cap and 3’ tail from an mRNA strand in a eukaryotic cell. What will likely happen?

The mRNA may be degraded before it can be translated in the cytoplasm.

A defective tRNA has a mismatched anticodon. How might this affect protein synthesis?

It could insert the wrong amino acid into the growing polypeptide, potentially altering protein function.

If a mutation causes an early stop codon in an mRNA sequence, what is the likely result?

Premature termination of translation, resulting in a shorter, likely nonfunctional protein.

A bacterial infection is treated with a drug that targets bacterial rRNA. Why does this not affect human cells?

Because bacterial ribosomes differ slightly from human ribosomes, allowing selective targeting.

During translation, what would happen if the ribosome fails to shift the mRNA forward after adding an amino acid?

Translation would stall because the A site wouldn't be available for the next tRNA.

What is a mutation?

A change in the genetic information of a cell.

What is a silent mutation?

A mutation where the substituted nucleotide still codes for the same amino acid, causing no change in protein function.

What is a missense mutation?

A mutation that changes one amino acid in the protein sequence.

What is a nonsense mutation?

A mutation that changes a codon into a stop codon, prematurely ending translation.

What is a frameshift mutation?

A mutation caused by the insertion or deletion of nucleotides not in multiples of three, altering the reading frame of the codons.

What are mutagens?

Physical or chemical agents that cause mutations.

What is a virus made of?

Nucleic acid (DNA or RNA) wrapped in a protein coat called a capsid; some also have a membranous envelope.

What is the lytic cycle?

A viral reproductive cycle where the virus takes over the host cell, produces new viruses, and causes the host to burst (lyse).

What is the lysogenic cycle?

A viral reproductive cycle where viral DNA is integrated into the host genome and replicated without killing the host.

What is the F factor in bacteria?

A specific piece of DNA that enables a bacterium to form sex pili and perform conjugation.

Why can frameshift mutations be more harmful than point mutations?

They shift the reading frame, changing every amino acid after the mutation, which often results in a completely nonfunctional protein.

How do silent mutations occur despite changes in nucleotide sequence?

Because of the redundancy in the genetic code; multiple codons can code for the same amino acid.

How do viruses reproduce if they are not cells?

They hijack host cells' machinery to replicate their genetic material and produce viral components.

How does the lysogenic cycle benefit the virus?

It allows the virus to persist in the host cell without destroying it, spreading through host cell divisions.

Why are bacteria useful in genetic studies?

They reproduce quickly, have simple genomes, and can exchange genes through transformation, transduction, and conjugation.

A DNA sequence experiences a nucleotide deletion not in multiples of three. What is the most likely outcome during translation?

A frameshift mutation occurs, likely producing a nonfunctional protein.

A patient is exposed to UV light causing DNA damage. What type of mutation might this cause and why?

UV light can act as a mutagen, potentially causing point mutations or thymine dimers leading to replication errors.

A bacterium gains the ability to produce a toxin after infection by a virus. Which process most likely caused this?

Transduction, where a bacteriophage transfers DNA from one bacterium to another.

A virus infects a cell but doesn't destroy it. Instead, its DNA integrates into the host chromosome. What viral cycle is this?

The lysogenic cycle.

A bacterial population suddenly acquires antibiotic resistance. Which gene transfer process likely occurred?

Conjugation, through which an F factor-bearing donor cell transferred a resistance gene to the recipient.