Podcast
Questions and Answers
Which type of mutation results in a change of an amino acid codon into a stop codon, nearly always leading to a nonfunctional protein?
What is the result of frameshift mutations?
Ribosomes require less than a minute to translate an average-sized mRNA into a polypeptide.
True
During termination, a release factor binds to the start codon and hydrolyzes the bond between the polypeptide and its tRNA in the P site.
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Translocation of tRNA involves the ribosome moving the tRNA with the attached polypeptide from the A site to the P site.
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During transcription, RNA polymerase adds nucleotides to the 5' end of the growing RNA strand.
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The 5' cap added to the pre-mRNA molecule protects mRNA from hydrolytic enzymes and serves as a translation start point for ribosomes.
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Eukaryotic cells modify pre-mRNA by adding a poly(A) tail to the 5' end of the molecule.
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What is the function of DNA ligase in DNA replication?
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What is the role of helicase in DNA replication?
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Why can't DNA polymerases add nucleotides to the 5' end of a growing DNA strand?
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Study Notes
DNA Replication
- Base pairing allows existing DNA strands to be templates for new complementary strands during replication.
- Nucleotides are arranged along the template strands according to base-pairing rules.
- Major enzymes involved in DNA replication include Helicase, Primase, Polymerase, and Ligase.
- E. coli can copy 5 million base pairs in less than an hour, while human cells can replicate 6 billion base pairs in a few hours with high accuracy, experiencing only one error per billion nucleotides.
Replication Mechanism
- DNA replication begins at the origin of replication, forming a replication bubble where strands separate.
- Replication proceeds bidirectionally, with multiple bubbles per chromosome in eukaryotes.
- Primers (short RNA segments) are created by Primase, enabling DNA polymerases to elongate the new DNA strands.
- New DNA strands elongate at the 3’ end, only in the 5’ to 3’ direction.
- Two types of strands are synthesized:
- Leading strand: Continuous synthesis towards the replication fork.
- Lagging strand: Discontinuous synthesis away from the fork into Okazaki fragments.
Steps of DNA Replication
- Helicase separates DNA strands, breaking hydrogen bonds at the replication fork.
- DNA Polymerase adds complementary nucleotides; bonds form covalently between phosphates and deoxyribose.
- Semi-conservative replication results in one original and one new DNA strand per double helix.
- Gaps on the lagging strand are joined by DNA Ligase.
RNA Types and Functions
- mRNA carries genetic information from DNA to the cytoplasm.
- rRNA is a key component of ribosomes.
- tRNA transports specific amino acids to ribosomes for polypeptide formation.
Gene Expression: Transcription and Translation
- Genetic information is encoded in nucleotide sequences, guiding protein synthesis.
- Transcription involves synthesizing mRNA from a DNA template, producing a messenger RNA molecule.
- Translation occurs at ribosomes, decoding mRNA to assemble polypeptide chains from amino acids.
- The processes are similar in prokaryotes and eukaryotes, with bacterial transcription and translation occurring simultaneously.
Transcription Process
- Stages: Initiation, Elongation, Termination.
- Initiation starts at the promoter, guiding RNA polymerase binding.
- RNA polymerase unwinds DNA, matching nucleotides to form RNA.
RNA Processing in Eukaryotes
- Modifications to pre-mRNA in the nucleus include adding a 5’ cap and a poly(A) tail for stability and export.
- RNA splicing removes non-coding regions (introns) to form mature mRNA.
Translation Mechanism
- tRNA picks up and delivers amino acids to the mRNA at ribosomes.
- Ribosomes consist of large and small subunits formed from rRNA and proteins.
- The process of translation is also segmented into initiation, elongation, and termination.
- Each ribosome has distinct binding sites for mRNA and tRNA to facilitate polypeptide formation.
Codons and the Genetic Code
- Codons (triplets of nucleotides) specify amino acids; the first codon (AUG) signals the start of translation.
- Each tRNA recognizes mRNA codons, linking specific amino acids to their corresponding codons based on their anticodons.
- Genetic instructions for polypeptide chains are written as triplet codes in DNA, which are subsequently translated into protein sequences.### RNA Processing and Protein Synthesis
- Primary transcripts undergo modifications (RNA processing) before exiting the nucleus to become finished mRNA.
- Genetic messages for protein synthesis are conveyed by messenger RNA (mRNA).
- The molecular chain of command: DNA → Transcription → mRNA → Translation → Protein.
- Genetic code consists of nucleotide triplets (codons) that specify amino acids.
- Each triplet of nucleotide bases represents the smallest coding unit for amino acids.
Transcription Process
- Transcription occurs in three stages: initiation, elongation, and termination.
- The promoter region marks the starting point for transcription and serves as a binding site for RNA polymerase.
- RNA polymerase synthesizes RNA by adding nucleotides to the 3’ end of the growing strand, following base-pairing rules where uracil pairs with adenine.
- A terminator sequence in DNA signals the end of transcription.
RNA Modification in Eukaryotes
- Eukaryotic cells modify pre-mRNA by adding a 5’ cap for protection and translation initiation, and a poly(A) tail (50 to 250 adenine nucleotides) to facilitate nuclear export.
- RNA splicing removes non-coding regions (introns) from pre-mRNA.
Translation Mechanism
- Translation is the synthesis of polypeptides directed by mRNA.
- Transfer RNA (tRNA) carries amino acids from the cytoplasm to the ribosome, matching codons on mRNA to the corresponding amino acids.
- Each tRNA has an anticodon that base-pairs with the mRNA codon.
- Codons on mRNA are read in the 5’ to 3’ direction.
Ribosome Structure and Function
- Ribosomes are composed of ribosomal RNA (rRNA) and proteins and consist of large and small subunits.
- Ribosomes have three tRNA binding sites:
- P site: Holds tRNA with the growing polypeptide.
- A site: Holds tRNA with the next amino acid.
- E site: Discharged tRNA exits.
Stages of Translation
- Initiation brings together mRNA, a tRNA (starting with methionine), and ribosomal subunits.
- Elongation involves a series of cycles, where each tRNA matches its anticodon to mRNA codons while contributing amino acids.
- Termination occurs when a stop codon is reached, signaling the end of polypeptide synthesis.
DNA Replication
- DNA replication is semiconservative, yielding two identical DNA strands with one original and one new strand.
- The replication fork is created by helicase, which unwinds the DNA double helix.
- DNA polymerases add nucleotides in a 5’ to 3’ direction, requiring a primer (short RNA segment).
- Leading strand synthesis is continuous, while the lagging strand is synthesized in Okazaki fragments joined by DNA ligase.
DNA Replication Efficiency
- E. coli replicates its 5 million base pairs in under an hour; human cells can replicate 6 billion base pairs within a few hours, with an accuracy of one error per billion nucleotides.
- Multiple origins of replication allow for rapid DNA copying in eukaryotes.
Summarizing Transcription and Translation
- Both processes connect genes to proteins, with RNA serving as the bridge between DNA and protein synthesis.
- Transcription occurs in eukaryotic nuclei, while translation mainly occurs at ribosomes in the cytoplasm.
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Description
Test your knowledge about the molecular chain of command in a cell, including DNA transcription, mRNA translation, and the genetic code's role in specifying amino acids. Understand the process of how genetic instructions for a polypeptide chain are written in DNA and transcribed into RNA.
