HMG - Week 11: Translation and Protein Synthesis

Export of mRNA to the Cytoplasm

  • Necessity of mRNA Export

    • In eukaryotic organisms, the processes of transcription and translation are spatially separated by the nuclear envelope.

    • Transcription occurs within the nucleus, while translation (protein synthesis) takes place in the cytoplasm.

    • Consequently, mature messenger RNA (mRNA) must be successfully transported through the nuclear envelope to reach the ribosomes for translation to commence.

  • Step-by-Step Process of mRNA Export

    1. Mature mRNA molecules are directed toward a structure known as the Nuclear Pore Complex (NPC).

    2. Proteins that were involved in the splicing process are removed from the mRNA transcript.

    3. Specific export proteins bind to the mRNA molecule.

    4. The mRNA complex docks onto the Nuclear Pore Complex.

    5. The mRNA is translocated through the pore, with the 5′ cap5' \text{ cap} leading the way.

    6. Export proteins dissociate from the mRNA once in the cytoplasm and are recycled back into the nucleus.

    7. The mRNA is released and is now available for the translation process.

  • The Nuclear Pore Complex (NPC)

    • The NPC is a substantial protein channel that facilitates communication between the nucleus and the cytoplasm.

    • Functions:

    • It acts as a gatekeeper controlling the movement of various molecules between the two compartments.

    • Small molecules are allowed to move passively through the channel.

    • Larger molecules require active transport mechanisms to pass through.

    • There are approximately 3,0003,000 NPCs distributed throughout the nuclear envelope.

    • Key Features:

    • The structure is cylindrical.

    • It exhibits eight-fold symmetry.

    • It possesses the capacity to transport molecules in both directions (into and out of the nucleus) simultaneously.

  • Export Proteins and Signals

    • mRNA strands contain specific export signals that are identified by export receptors.

    • Examples of Export Receptors:

    • HnRNP A1

    • HnRNP K

    • These proteins play a critical role in guiding the mRNA through the pore structure.

  • Transport Mechanisms

    • Ran-Dependent Export:

    • This mechanism requires an export receptor and the protein RanGTP.

    • Process:

      1. RanGTP facilitates the binding of export receptors to the mRNA.

      2. The entire molecular complex passes through the NPC.

      3. RanGTP is hydrolyzed/converted to RanGDP.

      4. The complex dissociates, resulting in the release of the mRNA.

    • Ran-Independent Export:

    • Most mRNA molecules utilize the TAP/NXT1 exporter complex.

    • Advantages:

      • This pathway does not require RanGTP.

      • It serves as the primary pathway for mRNA export in eukaryotic cells.

Chemical Structure of Proteins

  • Definition of Proteins

    • Proteins are biological polymers composed of amino acids linked together by peptide bonds.

  • General Structure of an Amino Acid

    • Every amino acid is composed of:

    • An amino group (−NH2-NH_2).

    • A carboxyl group (−COOH-COOH).

    • A central hydrogen atom.

    • A variable R-group, also known as the side chain, which determines the identity and properties of the amino acid.

  • Peptide Bond Formation

    • A peptide bond is created when the carboxyl group of one amino acid reacts with the amino group of another amino acid.

    • This reaction is a condensation reaction, leading to the release of a water (H2OH_2O) molecule.

  • Polypeptide Chains

    • A polypeptide is formed when many amino acids are joined together in a long chain.

    • A functional protein consists of one or more of these polypeptide chains folded into a specific shape.

  • Functions of Proteins in the Cell

    • Enzymes: Catalysts for biochemical reactions (e.g., Amylase).

    • Transport: Movement of substances (e.g., Haemoglobin transporting oxygen).

    • Structure: Providing physical support (e.g., Keratin in hair and nails).

    • Movement: Facilitating cellular and muscular motion (e.g., Actin and Myosin).

    • Signalling: Communication within and between cells (e.g., Hormones and their receptors).

    • Defence: Protecting the organism from pathogens (e.g., Antibodies).

Molecular Structure of Proteins

  • Levels of Protein Organization

    • Primary Structure:

    • This is the linear sequence of amino acids in the polypeptide chain.

    • The sequence is dictated directly by the nucleotide sequence in the DNA.

    • Example: Met-Ala-Gly-Lys-Val.

    • Secondary Structure:

    • Refers to local folding patterns of the polypeptide backbone caused by hydrogen bonds.

    • Two common types are the α-helix\alpha\text{-helix} and the β-pleated sheet\beta\text{-pleated sheet}.

    • Tertiary Structure:

    • This represents the overall three-dimensional folding of a single polypeptide chain.

    • It is stabilized by various interactions: hydrogen bonds, ionic bonds, hydrophobic interactions, and disulfide bonds.

    • Quaternary Structure:

    • This involves the association of multiple polypeptide subunits into a single functional unit.

    • Example: Haemoglobin, which is comprised of four distinct polypeptide subunits.

The Nature of the Genetic Code

  • Definition and Purpose

    • The genetic code is the set of rules relating nucleotide sequences in DNA/mRNA to the amino acid sequences in proteins.

    • It serves as the mechanism to convert information stored at the nucleic acid level into functional proteins.

  • Key Features of the Code

    • It utilizes four RNA bases: Adenine (A), Uracil (U), Guanine (G), and Cytosine (C).

    • The code is read in discrete groups of three bases.

    • Each triplet of bases specifies one particular amino acid.

  • The Triplet Code

    • Mathematical Basis:

    • With 44 possible RNA bases, using a triplet length yields 43=644^3 = 64 unique possible codons.

    • This quantity (6464) is sufficient to encode for all 2020 standard amino acids, as well as start and stop signals.

    • Codon Definition:

    • A codon is a sequence of 33 RNA nucleotides that corresponds to 11 amino acid.

    • Example: The codon AUG specifies the amino acid Methionine.

  • Reading Frame

    • Definition: The reading frame reflects the specific way codons are grouped and parsed during translation.

    • Example: Given the sequence AUGCCCGAA:

    • It is read as AUG | CCC | GAA.

    • If the starting point of the sequence is shifted, every subsequent codon is altered.

    • Frameshift Mutation:

    • Occurs when a nucleotide is inserted or deleted, shifting the reading frame.

    • Consequences:

      • Leads to the addition of entirely different amino acids.

      • Frequently results in the production of non-functional proteins.

  • Deciphering the Code

    • Scientific research established that:

    • Each codon specifies exactly one amino acid.

    • Codons are read sequentially one after the other.

    • There is no overlap between adjacent codons.

    • The Nirenberg Discovery:

    • Marshall Nirenberg and his colleagues proved that the codon UUU specifies the amino acid Phenylalanine.

    • This breakthrough was the first step in cracking the complete genetic code.

  • Essential Characteristics of the Genetic Code

    • Universal: Nearly all living organisms share the same genetic code. For instance, AUG codes for methionine in bacteria, plants, and humans alike.

    • Non-overlapping: Every nucleotide is a part of only one specific codon.

    • Continuous: There is no "punctuation" or gaps between codons; they are read back-to-back.

    • Unambiguous: A single codon will only ever specify one particular amino acid.

    • Degenerate: Multiple different codons can code for the same amino acid.

Degeneracy and Silent Mutations

  • Definition of Degeneracy

    • Since there are 6464 codons and only 2020 amino acids, most amino acids are specified by more than one codon.

    • Example: Leucine

    • Leucine can be coded by six different codons: UUA, UUG, CUU, CUC, CUA, and CUG.

  • Biological Importance

    • Degeneracy provides a protection mechanism against mutations.

    • Example: Glutamate

    • If a mutation changes GAA to GAG, both codons still specify the amino acid glutamate.

    • Consequently, the protein's overall amino acid sequence remains unchanged.

    • This type of mutation is referred to as a silent mutation.

Start and Stop Codons

  • Start Codon: AUG

    • Functions:

    1. Codes for the amino acid Methionine.

    2. Acts as the signal to initiate the process of translation.

    • The initiator tRNA possesses the anticodon UAC.

  • Stop Codons

    • These codons do not code for any amino acid.

    • There are three stop codons:

    1. UAA

    2. UAG

    3. UGA

    • Functions:

    • Signal the termination of protein synthesis.

    • Recruit release factors to the ribosome.

    • Facilitate the release of the completed polypeptide chain.

The Wobble Hypothesis

  • Proposed by Francis Crick

    • This hypothesis explains why cells require fewer unique tRNAs than there are codons.

  • The Principle of Wobble

    • While the first two bases of a codon must pair strictly with the tRNA anticodon, the third base of the codon exhibits flexibility in its pairing (the "wobble").

    • Example:

    • A single tRNA may recognize GGU, GGC, GGA, and GGG.

    • All four of these codons code for the amino acid Glycine.

  • Importance

    • It provides the physical basis for the degeneracy of the genetic code.

    • It reduces the total number of different tRNAs required by a cell.

Transfer RNA (tRNA) Structure and Function

  • General Function

    • tRNA acts as an adaptor molecule, bridging the gap between mRNA codons and the corresponding amino acids.

  • Structural Details

    • A tRNA molecule is typically between 7474 and 9595 nucleotides in length.

    • Key Components:

    • Acceptor arm: The site where the specific amino acid binds.

    • Anticodon arm: Contains the anticodon that pairs with the mRNA codon.

    • D loop

    • T loop

    • Variable loop

  • Anticodon Pairing

    • The anticodon is complementary and antiparallel to the mRNA codon.

    • Example: For an mRNA codon of AUG, the corresponding tRNA anticodon is UAC.

Ribosomes and Their Functional Sites

  • Function

    • Ribosomes are the physical sites where protein synthesis occurs.

  • Composition

    • Ribosomes are composed of ribosomal RNA (rRNA) and various proteins.

  • The Eukaryotic Ribosome

    • Small Subunit (40S40S): Consists of 18S18S rRNA.

    • Large Subunit (60S60S): Consists of 5S5S rRNA, 5.8S5.8S rRNA, and 28S28S rRNA.

  • Ribosomal Binding Sites

    • A site (Aminoacyl site): Accommodates the incoming aminoacyl-tRNA.

    • P site (Peptidyl site): Holds the tRNA linked to the growing polypeptide chain.

    • E site (Exit site): The location where empty (uncharged) tRNAs exit the ribosome.

Stages of Translation

  • Definition

    • Translation is the synthesis of a polypeptide chain using an mRNA strand as a template.

  • Stage 1: Initiation

    • Events:

    1. The small ribosomal subunit binds to the mRNA molecule.

    2. The initiator tRNA binds to the AUG start codon.

    3. Methionine is established as the first amino acid in the sequence.

    4. The large ribosomal subunit joins the complex.

    5. The complete initiation complex is formed.

    • Result: The initiator tRNA is positioned in the P site, while the A site remains vacant.

  • Stage 2: Elongation

    • This is a repeated cycle consisting of three core steps:

    • Step 1: Codon Recognition

      • A new aminoacyl-tRNA enters the A site.

      • Hydrogen bonding occurs between the codon and the anticodon.

    • Step 2: Peptide Bond Formation

      • The ribosome catalyzes the formation of a peptide bond.

      • The growing polypeptide chain is transferred to the amino acid currently in the A site.

    • Step 3: Translocation

      • The ribosome moves along the mRNA in the 5′→3′5' \rightarrow 3' direction.

      • The tRNAs shift positions: the tRNA in the A site moves to the P site, and the tRNA in the P site moves to the E site.

      • The empty tRNA exits from the E site.

    • The cycle repeats until a stop codon is encountered.

  • Stage 3: Termination

    • Events:

    1. A stop codon (UAA, UAG, or UGA) enters the A site.

    2. A release factor protein binds to the stop codon.

    3. The completed polypeptide is released from the ribosome.

    4. The ribosomal subunits dissociate from each other.

    5. Translation is concluded.

Exam Summary: Critical Concepts

  • Translation involves the conversion of mRNA information into protein.

  • A Codon consists of 33 bases and represents 11 amino acid.

  • The Start codon is AUG.

  • The Stop codons are UAA, UAG, and UGA.

  • The tRNA anticodon pairs with the mRNA codon through base pairing.

  • Functional sites: A site (incoming tRNA), P site (growing chain), E site (exit).

  • The ribosome moves in the 5′→3′5' \rightarrow 3' direction.

  • The stages of translation are Initiation, followed by Elongation, and then Termination.

  • The genetic code is characterized as universal, degenerate, non-overlapping, and unambiguous.

  • The Wobble hypothesis explains how one tRNA species can recognize multiple codons.