Molecular Biology - Central Dogma & Protein Synthesis Flashcards

Question 1

What is the Central Dogma of Molecular Biology?

Accepted Answer

The fundamental principle describing the flow of genetic information: DNA $\rightarrow$ RNA $\rightarrow$ Protein. It states that information cannot be transferred back from protein to nucleic acid, or from RNA to DNA (with exceptions like reverse transcription).

Question 2

What is the primary function of DNA replication?

Accepted Answer

To produce two identical copies of a DNA molecule from a single original DNA molecule, ensuring that each daughter cell receives a complete set of genetic material during cell division.

Question 3

Describe the semi-conservative nature of DNA replication.

Accepted Answer

Each new DNA molecule consists of one original (parental) strand and one newly synthesized (daughter) strand.

Question 4

Identify the key enzymes involved in unwinding and stabilizing DNA during replication initiation.

Accepted Answer

1. **DNA Helicase:** Unwinds the double helix by breaking hydrogen bonds between base pairs.
2. **DNA Topoisomerase (e.g., Gyrase):** Relieves supercoiling tension ahead of the replication fork.
3. **Single-Strand Binding Proteins (SSBPs):** Bind to and stabilize the separated single DNA strands, preventing them from re-annealing.

Question 5

What is the role of DNA Primase in DNA replication?

Accepted Answer

Synthesizes short RNA primers, which provide a free 3'-OH group that DNA polymerase can extend from, as DNA polymerase cannot initiate DNA synthesis de novo.

Question 6

Differentiate between DNA Polymerase III and DNA Polymerase I in prokaryotic DNA replication.

Accepted Answer

1. **DNA Polymerase III:** The primary enzyme for synthesizing new DNA strands during elongation, adding nucleotides in the 5' to 3' direction.
2. **DNA Polymerase I:** Removes RNA primers (5' to 3' exonuclease activity) and fills in the gaps with DNA (5' to 3' polymerase activity).

Question 7

Explain the formation of Okazaki fragments and their significance.

Accepted Answer

Short DNA segments synthesized discontinuously on the lagging strand during DNA replication. Because DNA polymerase can only synthesize in the 5' to 3' direction, the lagging strand must be synthesized in fragments as the replication fork opens. These fragments are later joined by DNA ligase.

Question 8

What is the function of DNA Ligase in DNA replication?

Accepted Answer

Joins the Okazaki fragments on the lagging strand, and any other nicks in the DNA backbone, by forming phosphodiester bonds.

Question 9

What is the primary purpose of transcription?

Accepted Answer

To synthesize an RNA molecule (mRNA, tRNA, or rRNA) from a DNA template, transferring genetic information from DNA to RNA.

Question 10

What is the main enzyme responsible for transcription, and in which direction does it synthesize RNA?

Accepted Answer

RNA Polymerase. It synthesizes RNA in the 5' to 3' direction, using the DNA template strand (3' to 5') as a guide.

Question 11

Define a promoter in the context of transcription.

Accepted Answer

A specific DNA sequence located upstream of a gene that serves as a binding site for RNA polymerase and transcription factors, initiating transcription.

Question 12

How do enhancers differ from promoters in regulating gene expression?

Accepted Answer

1. **Promoters:** DNA sequences located immediately upstream of a gene, essential for transcription initiation.
2. **Enhancers:** DNA sequences that can be located far from a gene (upstream, downstream, or within an intron) and increase the rate of transcription by binding activator proteins, often by looping the DNA to interact with the promoter region.

Question 13

List the three major post-transcriptional modifications that occur in eukaryotic mRNA.

Accepted Answer

1. **5' Capping:** Addition of a 7-methylguanosine cap to the 5' end.
2. **3' Polyadenylation:** Addition of a poly-A tail (multiple adenine nucleotides) to the 3' end.
3. **Splicing:** Removal of introns and ligation of exons.

Question 14

What is the function of the 5' cap and 3' poly-A tail on eukaryotic mRNA?

Accepted Answer

1. **5' Cap:** Protects mRNA from degradation, aids in nuclear export, and is crucial for ribosome binding during translation initiation.
2. **3' Poly-A Tail:** Protects mRNA from degradation, aids in nuclear export, and influences translation efficiency and mRNA stability.

Question 15

Explain the process of RNA splicing, including the roles of introns, exons, and the spliceosome.

Accepted Answer

Splicing is the removal of non-coding regions (introns) from a pre-mRNA molecule and the joining together of coding regions (exons). The spliceosome, a complex of small nuclear ribonucleoproteins (snRNPs) and other proteins, catalyzes this process.

Question 16

What is the genetic code, and what does it mean for it to be 'degenerate'?

Accepted Answer

The set of rules by which information encoded in genetic material (DNA or RNA sequences) is translated into proteins (amino acid sequences). It is 'degenerate' (or redundant) because most amino acids are specified by more than one codon.

Question 17

What are codons and anticodons, and how do they interact during translation?

Accepted Answer

1. **Codon:** A three-nucleotide sequence on mRNA that specifies a particular amino acid or a stop signal.
2. **Anticodon:** A three-nucleotide sequence on a tRNA molecule that is complementary to a specific mRNA codon.
They interact via complementary base pairing, ensuring the correct amino acid is delivered to the ribosome for incorporation into the polypeptide chain.

Question 18

Describe the role of tRNA in protein synthesis.

Accepted Answer

Transfer RNA (tRNA) molecules act as adaptors, carrying specific amino acids to the ribosome and recognizing the corresponding mRNA codons via their anticodon loop, thus ensuring the correct sequence of amino acids in the growing polypeptide chain.

Question 19

What are the three sites within a ribosome involved in translation, and what happens at each?

Accepted Answer

1. **A (Aminoacyl) site:** Binds incoming aminoacyl-tRNAs (tRNAs carrying an amino acid).
2. **P (Peptidyl) site:** Holds the tRNA carrying the growing polypeptide chain.
3. **E (Exit) site:** Where deacylated tRNAs (tRNAs that have released their amino acid) exit the ribosome.

Question 20

Outline the key events of translation initiation in eukaryotes.

Accepted Answer

The small ribosomal subunit binds to the 5' cap of the mRNA, then scans for the start codon (AUG). The initiator tRNA (carrying methionine) binds to the AUG codon in the P-site. Finally, the large ribosomal subunit joins, forming the complete initiation complex.

Question 21

Explain the process of peptide bond formation during translation elongation.

Accepted Answer

After an aminoacyl-tRNA binds to the A-site, the peptidyl transferase activity (a ribozyme activity of the large ribosomal subunit) catalyzes the formation of a peptide bond between the amino acid in the A-site and the growing polypeptide chain in the P-site. The polypeptide chain is then transferred to the tRNA in the A-site.

Question 22

How does translation termination occur?

Accepted Answer

When a stop codon (UAA, UAG, or UGA) enters the A-site, release factors (proteins) bind to it instead of a tRNA. This binding causes the hydrolysis of the bond between the polypeptide and the tRNA in the P-site, releasing the completed polypeptide chain. The ribosomal subunits then dissociate from the mRNA.

Molecular Biology - Central Dogma & Protein Synthesis

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