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Questions and Answers
What is the primary function of a promoter in transcription?
What is the primary function of a promoter in transcription?
- It terminates RNA synthesis.
- It serves as a binding site for RNA polymerase. (correct)
- It synthesizes ribonucleotides.
- It is responsible for DNA unwinding.
During which phase of transcription does RNA polymerase add nucleotides to the mRNA strand?
During which phase of transcription does RNA polymerase add nucleotides to the mRNA strand?
- Termination
- Reattachment
- Elongation (correct)
- Initiation
What marks the termination of transcription?
What marks the termination of transcription?
- The binding of RNA polymerase to a promoter.
- The addition of nucleotides at the 5' end.
- The re-winding of the DNA double helix.
- The formation of a hairpin loop followed by a run of U's. (correct)
What is the role of consensus sequences in relation to promoters?
What is the role of consensus sequences in relation to promoters?
How is RNA synthesized in relation to the DNA template?
How is RNA synthesized in relation to the DNA template?
What is the role of the Shine-Delgarno sequence in prokaryotic translation?
What is the role of the Shine-Delgarno sequence in prokaryotic translation?
Which component of the ribosome contains the 16S rRNA in prokaryotes?
Which component of the ribosome contains the 16S rRNA in prokaryotes?
What is N-formylmethionine (fMet) associated with during translation initiation?
What is N-formylmethionine (fMet) associated with during translation initiation?
Which statement accurately describes the role of elongation factors during translation?
Which statement accurately describes the role of elongation factors during translation?
What happens during the termination phase of translation?
What happens during the termination phase of translation?
What does the CsCl centrifugation banding pattern demonstrate about DNA replication?
What does the CsCl centrifugation banding pattern demonstrate about DNA replication?
Which phrase accurately describes the relationship between parental and daughter strands during DNA replication?
Which phrase accurately describes the relationship between parental and daughter strands during DNA replication?
Which of the following is NOT a phase of DNA replication?
Which of the following is NOT a phase of DNA replication?
What initiates the unwinding of bacterial DNA at the origin of replication?
What initiates the unwinding of bacterial DNA at the origin of replication?
What is the function of helicase in DNA replication?
What is the function of helicase in DNA replication?
What is formed at the termination region during DNA replication?
What is formed at the termination region during DNA replication?
What happens to the replication forks as DNA unwinds?
What happens to the replication forks as DNA unwinds?
What length is the origin of replication (oriC) in E.coli?
What length is the origin of replication (oriC) in E.coli?
What is required for DNA polymerase to extend a DNA chain?
What is required for DNA polymerase to extend a DNA chain?
Which enzyme is responsible for replacing RNA primers with DNA during DNA replication?
Which enzyme is responsible for replacing RNA primers with DNA during DNA replication?
How are Okazaki fragments synthesized during DNA replication?
How are Okazaki fragments synthesized during DNA replication?
At what sites does the termination of DNA replication occur?
At what sites does the termination of DNA replication occur?
What is the primary reason for the high accuracy of DNA replication?
What is the primary reason for the high accuracy of DNA replication?
In which direction does DNA polymerase III add nucleotides during DNA elongation?
In which direction does DNA polymerase III add nucleotides during DNA elongation?
What role do helicases play during DNA replication termination?
What role do helicases play during DNA replication termination?
Which statement about the template strands in DNA replication is correct?
Which statement about the template strands in DNA replication is correct?
What is the key feature of DNA polymerase enzymes that enables proofreading?
What is the key feature of DNA polymerase enzymes that enables proofreading?
Which component is different in RNA compared to DNA?
Which component is different in RNA compared to DNA?
What is the role of RNA polymerase during RNA synthesis?
What is the role of RNA polymerase during RNA synthesis?
What replaces thymine in RNA?
What replaces thymine in RNA?
How do RNA transcripts convey information from DNA?
How do RNA transcripts convey information from DNA?
What type of nucleotides do RNA molecules contain?
What type of nucleotides do RNA molecules contain?
When synthesizing RNA, which nitrogenous base pairs with adenine?
When synthesizing RNA, which nitrogenous base pairs with adenine?
Which statement is true about the bases in RNA compared to DNA?
Which statement is true about the bases in RNA compared to DNA?
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Study Notes
Meselson-Stahl Experiment
- CsCl centrifugation banding patterns demonstrated semi-conservative DNA replication.
DNA Replication Attributes (All Organisms)
- Parental DNA strands remain intact during replication.
- Each parental strand serves as a template for a complementary daughter strand.
- Replication yields two identical daughter duplexes, each with one parental and one daughter strand.
Three Phases of Replication
- Initiation
- Elongation
- Termination
Initiation of Replication
- Bacterial chromosomes have a single origin of replication (ori).
- Localized DNA unwinding starts at the origin.
- E. coli oriC is 245 base pairs long.
- DnaA, DnaB, and DnaC proteins (helicase complex) initiate unwinding by breaking hydrogen bonds.
- Helicase unwinds DNA further, creating a replication fork.
Bidirectional Replication
- Two replication forks move away from the origin, forming a replication bubble.
- Forks merge at the termination region (ter), completing replication of one replicon.
Elongation
- DNA elongation proceeds from the initiation bubble.
- DNA polymerases I and III are involved.
- DNA polymerase cannot initiate chains; it extends pre-existing chains.
- RNA primers, synthesized by primase in primosomes, initiate DNA synthesis.
- Nucleotide addition by DNA polymerase III is always at the 3' end, but proceeds in opposite directions on the two strands.
- Only one strand (leading strand) is synthesized continuously in the direction of the replication fork.
Lagging Strand Elongation
- Synthesized discontinuously as Okazaki fragments.
- Each Okazaki fragment requires its own RNA primer.
- Primosomes synthesize RNA primers.
- DNA polymerase I replaces RNA primers with DNA.
- DNA ligase joins Okazaki fragments.
Termination
- Occurs at ter sites, approximately 180° from the ori.
- Involves inhibiting helicases and separating completed chromosomes.
DNA Proofreading
- DNA polymerases proofread, correcting replication errors.
- E. coli has approximately one error per 100 million nucleotides.
- Proofreading is due to 3' to 5' exonuclease activity of DNA polymerases.
Transcription
- RNA transcripts carry genetic information.
RNA Nucleotides and Structure
- RNA ribonucleotides contain ribose sugar (instead of deoxyribose), adenine, guanine, cytosine, and uracil (instead of thymine).
- RNA synthesis uses complementary base pairing (A with U, C with G).
- RNA polymerase catalyzes ribonucleotide addition to the 3' end.
Bacterial Promoter Recognition
- Promoters are double-stranded DNA sequences where RNA polymerase binds.
- They're essential for transcription initiation.
- Located upstream of the coding sequence.
- Consensus sequences attract RNA polymerase.
Transcription Steps
- Initiation: RNA polymerase binds to the promoter, unwinding DNA at the gene's start.
- Elongation: Nucleotides are added to the 3' end, following complementary base pairing with the DNA template. RNA polymerase advances, and the DNA rewinds after transcription.
- Termination: Occurs at the 3' end of the transcript beyond the coding sequence (3' UTR). RNA polymerase recognizes termination signals (e.g., a GC-rich hairpin loop followed by several adenines), leading to its release and transcription termination.
Role of Ribosome and rRNA in Translation (Prokaryotes)
- Ribosomes are ~20 nm in diameter.
- Composed of 65% rRNA and 35% ribosomal proteins (RNP).
- Large subunit contains 23S and 5S rRNA.
- Small subunit contains 16S rRNA.
Stages of Translation
- Initiation: Ribosome recognizes mRNA (Shine-Dalgarno sequence in prokaryotes helps align the ribosome with the initiation codon). Initiator tRNAMet binds to the AUG start codon; methionine is formylated (fMet) in prokaryotes.
- Elongation: Elongation factors guide tRNA binding and ribosome movement.
- Termination: Ribosome pauses at a stop codon. A release factor binds, releasing the polypeptide, and the ribosome complex dissociates.
tRNA Binding Sites on Ribosome
-
A site (aminoacyl site): Accepts incoming charged tRNA.
-
P site (peptidyl site): Peptide bond formation.
-
E site (exit site): tRNA exits.
-
Amino terminus is synthesized first, near the 5' end of mRNA.
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