Molecular Biology Module 7: The Genetic Code

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Questions and Answers

How many stop codons are present in the genetic code?

3 (correct)

Which amino acid does the start codon AUG code for?

Methionine (correct)

Arginine

Leucine

Serine

Why is the genetic code described as degenerate?

It only contains three stop codons.

Many amino acids are specified by multiple codons. (correct)

Some codons do not code for amino acids.

There are more amino acids than codons.

What is the primary function of tRNA?

To link amino acids to their corresponding codons. Signup and view all the answers

What paradox is associated with tRNA?

Some tRNAs recognize more than one codon. Signup and view all the answers

What is the role of aminoacyl-tRNA synthetases?

To match tRNA with its corresponding amino acid. Signup and view all the answers

Which of the following statements about the anticodon is true?

It interacts with the codon in mRNA. Signup and view all the answers

How many total codons exist in the genetic code?

64 Signup and view all the answers

What does degeneracy in the genetic code refer to?

Multiple codons coding for a single amino acid Signup and view all the answers

What is the significance of wobble base pairing in tRNA?

It allows flexibility in codon-anticodon interactions Signup and view all the answers

Which of the following describes the role of inosine in tRNA?

It enables pairing with A, C, and U Signup and view all the answers

How many different tRNA species are there for certain amino acids due to genetic code degeneracy?

More than one tRNA species for some amino acids Signup and view all the answers

What does the term 'reading frame' refer to in the context of genetic translation?

The specific codon sequence that determines amino acid order Signup and view all the answers

What is the term used for a sequence of three nucleotides in mRNA that codes for an amino acid?

Codon Signup and view all the answers

How many different amino acids can be coded by a three-nucleotide (triplet) code?

64 Signup and view all the answers

What mechanism allows for the degeneracy of the genetic code?

Wobble pairing Signup and view all the answers

Which amino acid is represented by the codon 'UUU'?

Phenylalanine Signup and view all the answers

What is the significance of stop codons in the genetic code?

They terminate protein synthesis Signup and view all the answers

Which of the following scientists is NOT associated with the breaking of the genetic code?

James Watson Signup and view all the answers

What is the result when the triplet code UUU is repeated in a polynucleotide?

Produces a polypeptide containing phenylalanine Signup and view all the answers

How many codons do not code for any amino acids?

3 Signup and view all the answers

What is the function of tRNA in translation?

It brings amino acids to the ribosome Signup and view all the answers

What is the main outcome of the Nirenberg-Matthaei experiment?

Identification of codon-amino acid pairs Signup and view all the answers

What are the three main stages of translation?

Initiation, Elongation, Termination Signup and view all the answers

What is the primary role of mRNA in the context of protein synthesis?

To code for a polypeptide chain Signup and view all the answers

Which statement correctly describes the ribosome's structure?

It is an RNA/protein complex composed of two subunits. Signup and view all the answers

What role do cofactors play in the translation process?

They provide the energy needed for translation. Signup and view all the answers

What is the significance of the large and small subunits of the ribosome?

They participate in the binding of tRNA and mRNA. Signup and view all the answers

What plays a crucial role in identifying the site of initiation of protein synthesis in prokaryotes?

Shine-Dalgarno sequence Signup and view all the answers

Which molecule is the formylated version of methionine used in prokaryotic translation initiation?

fMet Signup and view all the answers

What is the process called in which amino acids are attached to tRNA?

Amino acid activation Signup and view all the answers

Which component interacts with the 30S subunit during the initiation of translation in prokaryotes?

Initiator tRNA Signup and view all the answers

In the amino acid activation process, what is the first product formed when an amino acid reacts with ATP?

AA-AMP Signup and view all the answers

What completes the initiation complex in translation after the large subunit joins?

mRNA binding Signup and view all the answers

What is the role of active IF2 during translation initiation?

Facilitates tRNA binding Signup and view all the answers

In eukaryotes, what is the equivalent of the Shine-Dalgarno sequence for identifying the initiation site?

Kozak sequence Signup and view all the answers

What is the primary role of IF3 during the initiation of translation?

Stabilizing the free 30S ribosomal subunit Signup and view all the answers

In what way does EF-Tu function during the elongation phase of translation?

It facilitates the entry of aminoacyl-tRNA into the ribosome Signup and view all the answers

What is the significance of the ternary complex during translation?

It consists of aminoacyl-tRNA, EFTu, and GTP Signup and view all the answers

Which factor is responsible for terminating translation when a termination codon appears?

RF-GTP Signup and view all the answers

What is the role of GTP in protein synthesis?

It hydrolyzes to GDP to release energy Signup and view all the answers

Which statement about the P site during translation is correct?

It is occupied by tRNA carrying the polypeptide chain Signup and view all the answers

What occurs during ribosome translocation in the elongation stage of translation?

Empty tRNA is ejected from the A site Signup and view all the answers

What is unique about tRNA-Met during the initiation of translation?

It is the only tRNA that can bind to the P site Signup and view all the answers

Which release factors play a role in the termination of translation?

RF1, RF2, and RF3 Signup and view all the answers

What does the role of IF2 include during the initiation of translation?

Tagging tRNA for entry into the ribosome Signup and view all the answers

Study Notes

Molecular Biology - Module 7, Lecture 1: The Genetic Code

The lecture focuses on the genetic code, how it works, and how it was discovered.
Proteins are made of 20 amino acids, but messenger RNA (mRNA) only has 4 nucleotides.
A sequence of 3 nucleotides (a codon) specifies a particular amino acid.
There are 64 possible codons (4^3).
61 of these codons specify amino acids; the remaining 3 are stop codons.
The genetic code is degenerate, meaning multiple codons can code for the same amino acid.
This degeneracy is facilitated by wobble base pairing in transfer RNA (tRNA).

Aims of the Lecture

Degeneracy of the genetic code.
How the genetic code was broken (Nirenberg-Matthaei experiment).
Reading frames.
The role of tRNA.
Aminoacyl-tRNA synthesis.
Wobble, a mechanism for degeneracy.

The Genetic Code

Proteins are made up of 20 amino acids.
mRNA only has 4 nucleotides.
A three-nucleotide sequence (codon) is required to code for each amino acid.
There are 16 possible combinations of 2 nucleotides.
There are 64 possible combinations of 3 nucleotides (codons).
61 of those 64 codons specify amino acids.
The remaining three codons signal the end of the protein sequence (stop codons).

The Nirenberg-Matthaei Experiment

This experiment was crucial in deciphering the genetic code.
Researchers created synthetic mRNA with only one nucleotide (e.g., UUUUUUU).
The resulting polypeptide chain contained only one amino acid (e.g., phenylalanine), demonstrating a direct relationship between mRNA sequence and amino acid sequence. This was a critical step in breaking the genetic code.

Reading Frames

A reading frame is a sequence of codons that runs from a specific start codon to a specific stop codon.
Almost all protein chains begin with AUG (methionine).
There are three possible reading frames for any sequence of nucleotides.
Incorrect reading frame shifts will change the sequence, often leading to non-functional proteins. This is a significant aspect affecting protein function.

Transfer RNA (tRNA)

tRNA interacts with mRNA codons via an anticodon loop.
tRNA has an amino acid acceptor arm where the appropriate amino acid is attached ensuring the correct amino acid is added to the chain.
The anticodon loop of tRNA binds to the mRNA codon, thus facilitating accurate protein synthesis.
tRNA is essential for linking mRNA codons to their corresponding amino acids during protein synthesis.

Aminoacyl-tRNA Synthetases

Each tRNA is recognized by a specific aminoacyl-tRNA synthetase.
These enzymes attach the correct amino acid to its corresponding tRNA. These enzymes ensure accuracy in pairing amino acids and tRNA, essential for correct protein sequence.
The synthesis process is highly specific to ensure accuracy.
There are more tRNAs than amino acids since some tRNAs can recognize more than one codon.

Wobble Hypothesis

Wobble allows unconventional base pairing between the 3rd base of a codon and the 1st base of the anticodon. In other words, irregular base pairing.
This flexibility helps to accommodate the redundancy of the genetic code. This is how multiple codons can specify the same amino acid; explains why some tRNAs can recognize more than one codon.

Summary

A degenerate triplet genetic code translates nucleotide sequences into amino acid sequences.
Specificity in protein synthesis comes from amino acid-tRNA interactions.
Degeneracy means multiple codons for the same amino acid.
This redundancy is accomplished via wobble base pairing, a mechanism facilitating more than one tRNA to recognize more than one codon and accounting for the multiple codons for each amino acid.

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Description

This lecture covers the intricacies of the genetic code, detailing how proteins are synthesized from amino acids and the significance of codons. It explores the concept of degeneracy in the genetic code and the role of tRNA in the decoding process, highlighting the methods used to break the genetic code. Learn about reading frames, wobble base pairing, and aminoacyl tRNA synthesis.