Molecular Biology - Central Dogma & Gene Expression Flashcards

Question 1

What is the Central Dogma of Molecular Biology?

Accepted Answer

The Central Dogma describes the flow of genetic information within a biological system: DNA is replicated, DNA is transcribed into RNA, and RNA is translated into protein. It can be summarized as DNA $\rightarrow$ RNA $\rightarrow$ Protein.

Question 2

What is the primary purpose of DNA replication?

Accepted Answer

The primary purpose of DNA replication is to produce two identical copies of a DNA molecule from one original DNA molecule, ensuring that each daughter cell receives a complete and accurate set of genetic material during cell division.

Question 3

Describe the semi-conservative nature of DNA replication.

Accepted Answer

DNA replication is semi-conservative, meaning that each new DNA molecule consists of one original (parental) strand and one newly synthesized strand. This mechanism ensures high fidelity in genetic information transfer.

Question 4

What is the function of DNA helicase in replication?

Accepted Answer

DNA helicase is an enzyme that unwinds and separates the two complementary strands of the DNA double helix by breaking the hydrogen bonds between base pairs, creating a replication fork.

Question 5

What is the main role of DNA polymerase in replication?

Accepted Answer

DNA polymerase is the primary enzyme responsible for synthesizing new DNA strands. It adds nucleotides one by one to the growing DNA chain, complementary to the template strand, and also possesses proofreading activity to correct errors.

Question 6

What is the function of DNA ligase in DNA replication?

Accepted Answer

DNA ligase is an enzyme that catalyzes the formation of phosphodiester bonds between adjacent nucleotides, effectively joining DNA fragments (e.g., Okazaki fragments on the lagging strand) to create a continuous strand.

Question 7

Why is primase essential for DNA replication?

Accepted Answer

Primase is an RNA polymerase that synthesizes short RNA primers. DNA polymerase cannot initiate DNA synthesis de novo; it can only add nucleotides to an existing 3'-OH group. Primers provide this starting point for DNA polymerase.

Question 8

Differentiate between the leading and lagging strands during DNA replication.

Accepted Answer

The **leading strand** is synthesized continuously in the 5' to 3' direction, moving towards the replication fork. The **lagging strand** is synthesized discontinuously in short segments (Okazaki fragments) in the 5' to 3' direction, moving away from the replication fork, requiring multiple primers.

Question 9

What are Okazaki fragments and on which strand are they formed?

Accepted Answer

Okazaki fragments are short sequences of DNA nucleotides (and RNA primer) that are synthesized discontinuously on the **lagging strand** during DNA replication. These fragments are later joined together by DNA ligase.

Question 10

Briefly describe the initiation phase of DNA replication.

Accepted Answer

Initiation begins at specific DNA sequences called origins of replication. Initiator proteins bind to these sites, recruiting helicase to unwind the DNA and form replication bubbles, which then expand bidirectionally.

Question 11

Briefly describe the termination phase of DNA replication.

Accepted Answer

Termination occurs when replication forks meet or when specific termination sequences are encountered. In eukaryotes, this often involves the fusion of multiple replication bubbles. In prokaryotes, it's typically when the two forks meet at a specific termination site.

Question 12

List two key differences between prokaryotic and eukaryotic DNA replication.

Accepted Answer

1. **Origins of Replication:** Prokaryotes typically have a single origin of replication on their circular chromosome, while eukaryotes have multiple origins on each linear chromosome.
2. **Telomeres:** Eukaryotic chromosomes have telomeres at their ends, which require specialized replication mechanisms (telomerase) to prevent shortening; prokaryotes do not have telomeres.

Question 13

What is the primary purpose of transcription?

Accepted Answer

The primary purpose of transcription is to synthesize an RNA molecule (mRNA, tRNA, or rRNA) from a DNA template. For protein-coding genes, this mRNA molecule carries the genetic information from DNA to the ribosomes for protein synthesis.

Question 14

What is the main enzyme responsible for transcription?

Accepted Answer

The main enzyme responsible for transcription is **RNA polymerase**. It synthesizes an RNA strand complementary to a DNA template strand, without the need for a primer.

Question 15

Define the roles of promoter and terminator regions in transcription.

Accepted Answer

The **promoter** is a DNA sequence located upstream of a gene that signals where transcription should begin and to which RNA polymerase binds. The **terminator** is a DNA sequence that signals the end of transcription, causing RNA polymerase to detach from the DNA template.

Question 16

Briefly outline the three main stages of transcription.

Accepted Answer

1. **Initiation:** RNA polymerase binds to the promoter, unwinds the DNA, and begins synthesizing RNA.
2. **Elongation:** RNA polymerase moves along the DNA template, extending the RNA strand by adding complementary ribonucleotides.
3. **Termination:** RNA polymerase encounters a terminator sequence, causing it to detach from the DNA and release the newly synthesized RNA.

Question 17

List two key differences between prokaryotic and eukaryotic transcription.

Accepted Answer

1. **Location:** In prokaryotes, transcription and translation can occur simultaneously in the cytoplasm (coupled). In eukaryotes, transcription occurs in the nucleus, and translation occurs in the cytoplasm.
2. **RNA Processing:** Eukaryotic primary RNA transcripts (pre-mRNA) undergo extensive post-transcriptional modifications (splicing, 5' capping, 3' polyadenylation) before translation; prokaryotic mRNA typically does not.

Question 18

What is the primary purpose of translation?

Accepted Answer

The primary purpose of translation is to synthesize a polypeptide (protein) chain using the genetic information encoded in an mRNA molecule. This process converts the nucleotide sequence of mRNA into the amino acid sequence of a protein.

Question 19

What is the genetic code and what are its key characteristics (e.g., degeneracy)?

Accepted Answer

The genetic code is the set of rules by which information encoded in genetic material (DNA or RNA sequences) is translated into proteins (amino acid sequences). Key characteristics include:
- **Triplet:** Each codon consists of three nucleotides.
- **Non-overlapping:** Codons are read sequentially without overlap.
- **Degenerate (Redundant):** Most amino acids are specified by more than one codon.
- **Universal:** Largely the same across all forms of life.

Question 20

Describe the role of ribosomes in translation.

Accepted Answer

Ribosomes are complex molecular machines (composed of rRNA and proteins) that serve as the site of protein synthesis. They facilitate the binding of mRNA and tRNAs, catalyze the formation of peptide bonds between amino acids, and move along the mRNA during elongation.

Question 21

What is the function of tRNA in translation?

Accepted Answer

Transfer RNA (tRNA) molecules act as adaptors, carrying specific amino acids to the ribosome during translation. Each tRNA has an anticodon loop that base-pairs with a complementary codon on the mRNA, ensuring the correct amino acid is added to the growing polypeptide chain.

Question 22

Identify the start codon and the three stop codons in translation.

Accepted Answer

The **start codon** is AUG, which codes for Methionine (Met) and signals the beginning of translation. The three **stop codons** are UAA, UAG, and UGA, which do not code for any amino acid but instead signal the termination of translation.

Question 23

Briefly outline the three main stages of translation.

Accepted Answer

1. **Initiation:** The ribosome assembles around the mRNA start codon (AUG) and the initiator tRNA carrying Methionine.
2. **Elongation:** tRNAs bring amino acids to the ribosome, peptide bonds form between adjacent amino acids, and the ribosome moves along the mRNA.
3. **Termination:** The ribosome encounters a stop codon, release factors bind, and the polypeptide chain is released, followed by ribosome disassembly.

Question 24

List two key differences between prokaryotic and eukaryotic translation.

Accepted Answer

1. **Coupling:** In prokaryotes, transcription and translation are often coupled (occur simultaneously). In eukaryotes, they are spatially and temporally separated (transcription in nucleus, translation in cytoplasm).
2. **Initiation:** Eukaryotic translation initiation involves scanning for the 5' cap and the first AUG codon, while prokaryotic initiation involves the Shine-Dalgarno sequence upstream of the start codon to recruit the ribosome.

Question 25

What is gene expression?

Accepted Answer

Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product, such as a protein or a functional RNA (e.g., tRNA, rRNA). It encompasses transcription and translation, as well as regulatory mechanisms that control these processes.

Molecular Biology - Central Dogma & Gene Expression

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