2.28.2 Translation

Definition: Translation is the second step of gene expression. The mRNA synthesized in the nucleus of the cell moves into the cytoplasm where it binds to the ribosome. The mRNA codes are translated by ribosomes with the help of tRNAs of opposite anticodon, thus synthesizing a protein with specific amino acid sequence.

In simple words: Translation is the process by which the mRNA sequence is used to build a protein.

Key Points:

• Second step of gene expression.
• The mRNA synthesized in the nucleus moves into the cytoplasm.
• mRNA binds to the ribosome.
• The mRNA codes are translated by ribosomes with the help of tRNAs of opposite anticodon.
• This synthesizes a protein with a specific amino acid sequence.
• In this process, each set of 3 mRNA nucleotides called a codon specifies an amino acid in the polypeptide.
• Each cell has many different types of tRNAs for transferring each type of amino acid to ribosomes when needed.
• During protein synthesis by the ribosome, amino acids are linked together by peptide bonds to form a polypeptide chain or protein.

Components of Translation:

Steps of Translation:

• mRNA binds to the small ribosomal subunit.
• The start codon (AUG) is recognized.
• tRNA with the complementary anticodon (UAC) brings methionine.
• The large ribosomal subunit joins.

• Ribosome moves along mRNA.
• tRNA brings amino acids.
• Peptide bonds form between amino acids.
• The polypeptide chain grows.

• Ribosome reaches a stop codon (UAA, UAG, UGA).
• The polypeptide is released.
• Ribosome dissociates.

Base Pairing During Translation:

The Central Dogma: A Summary

DNA Transcription→ RNA Translation→ Protein

• DNA stores genetic information.
• RNA carries the genetic information.
• Protein is the functional product.
• Transcription occurs in the nucleus.
• Translation occurs in the cytoplasm (on ribosomes).
• Gene expression = Transcription + Translation.

Exceptions to the Central Dogma

Note: Reverse transcription is an exception to the Central Dogma. It occurs in retroviruses, where the enzyme reverse transcriptase converts RNA into DNA.

Intermediate Level

Transcription in Detail

Translation in Detail

Comparison of Transcription and Translation

Codons and Anticodons

The Genetic Code

• 64 codons in total.
• 61 codons code for amino acids.
• 3 codons are stop codons (UAA, UAG, UGA).
• AUG is the start codon (Methionine).
• The genetic code is universal (same in almost all organisms).
• The genetic code is degenerate (more than one codon can code for the same amino acid).

Important Points

• 1. Genes contain information needed for the synthesis of proteins molecules.
• 2. The flow of biological information, from DNA to RNA and from RNA to protein is known as the "central dogma of life".
• 3. Protein synthesis takes place in two major steps:
• - Transcription
• - Translation
• 4. Together transcription and translation are known as gene expression.
• 5. Transcription is the first step of gene expression.
• 6. When a new protein is needed by a cell, the two DNA strands of a gene that codes for a protein unwind from each other.
• 7. A single strand of messenger RNA (mRNA) is then synthesized by pairing up complementary mRNA nucleotides (adenine, cytosine, guanine, and uracil) with the exposed DNA (non-coding) strand.
• 8. After the segment is copied as mRNA, the both strands of DNA are "zipped" back together into its normal shape.
• 9. Uracil is paired in mRNA as complementary to adenine instead of thymine.
• 10. Translation is the second step of gene expression.
• 11. The mRNA synthesized in the nucleus of the cell moves into the cytoplasm where it binds to the ribosome.
• 12. The mRNA codes are translated by ribosomes with the help of tRNAs of opposite anticodon, thus synthesizing a protein with specific amino acid sequence.
• 13. In this process, each set of 3 mRNA nucleotides called a codon specifies an amino acid in polypeptide.
• 14. Each cell has many different types of tRNAs for transferring each type of amino acid to ribosomes when needed.
• 15. During protein synthesis by ribosome, amino acids are linked together by peptide bond to form a polypeptide chain or protein.

Remember

• - Central Dogma = DNA → RNA → Protein
• - **Transcription** = DNA → RNA
• - **Translation** = RNA → Protein
• - Transcription occurs in the nucleus.
• - Translation occurs in the cytoplasm (on ribosomes).
• - Gene expression = Transcription + Translation.
• - Transcription uses RNA polymerase.
• - Translation uses ribosomes, tRNA, and amino acids.
• - Codons = triplets on mRNA.
• - Anticodons = triplets on tRNA.
• - Start codon = AUG (Methionine).
• - **Stop codons** = UAA, UAG, UGA.
• - Reverse transcription = RNA → DNA (in retroviruses).

Common Mistakes

• 1. Confusing transcription and translation – Transcription = DNA → RNA; Translation = RNA → Protein.
• 2. Thinking transcription occurs in the cytoplasm – Transcription occurs in the nucleus.
• 3. Thinking translation occurs in the nucleus – Translation occurs in the cytoplasm (on ribosomes).
• 4. Confusing codons and anticodons – Codons are on mRNA; anticodons are on tRNA.
• 5. Forgetting that uracil replaces thymine in RNA – In RNA, U pairs with A.
• 6. Thinking the Central Dogma is reversible – The Central Dogma is unidirectional under normal conditions.
• 7. Forgetting the exceptions – Reverse transcription (RNA → DNA) occurs in retroviruses.
• 8. Confusing the start and stop codons – Start codon = AUG; Stop codons = UAA, UAG, UGA.
• 9. Thinking all codons code for amino acids – 61 codons code for amino acids; 3 are stop codons.
• 10. Forgetting that the genetic code is degenerate – More than one codon can code for the same amino acid.
• 11. Thinking transcription and translation are the same – They are different processes: transcription makes RNA; translation makes protein.
• 12. Forgetting the role of tRNA – tRNA brings amino acids to the ribosome.
• 13. Thinking mRNA is stable – mRNA is less stable than rRNA and tRNA.
• 14. Confusing gene expression with the Central Dogma – Gene expression = transcription + translation; Central Dogma = DNA → RNA → Protein.

Quick Revision

• Genes contain information needed for the synthesis of proteins molecules.
• The flow of biological information, from DNA to RNA and from RNA to protein is known as the "central dogma of life".
• Protein synthesis takes place in two major steps i.e., transcription and translation.
• Together transcription and translation are known as gene expression.
• Transcription is the first step of gene expression.
• When a new protein is needed by a cell, the two DNA strands of a gene that codes for a protein unwind from each other.
• A single strand of messenger RNA (mRNA) is then synthesized by pairing up complementary mRNA nucleotides (adenine, cytosine, guanine and uracil) with the exposed DNA (non-coding) strand.
• After the segment is copied as mRNA, the both strands of DNA are "zipped" back together into its normal shape.
• Uracil is paired in mRNA as complementary to adenine instead of thymine.
• Translation is the second step of gene expression.
• The mRNA synthesized in the nucleus of the cell moves into the cytoplasm where it binds to the ribosome.
• The mRNA codes are translated by ribosomes with the help of tRNAs of opposite anticodon, thus synthesizing a protein with specific amino acid sequence.
• In this process, each set of 3 mRNA nucleotides called a codon specify an amino acid in polypeptide.
• Each cell has many different types of tRNAs for transferring each type of amino acid to ribosomes when needed.
• During protein synthesis by ribosome, amino acids are linked together by peptide bond to form a polypeptide chain or protein.

60-Second Revision

• Central Dogma = DNA → RNA → Protein
• **Transcription** = DNA → RNA
• **Translation** = RNA → Protein
• Transcription occurs in the nucleus.
• Translation occurs in the cytoplasm (on ribosomes).
• Gene expression = Transcription + Translation.
• Transcription uses RNA polymerase.
• Translation uses ribosomes, tRNA, and amino acids.
• Codons = triplets on mRNA.
• Anticodons = triplets on tRNA.
• Start codon = AUG (Methionine).
• **Stop codons** = UAA, UAG, UGA.
• Reverse transcription = RNA → DNA (in retroviruses).
• Warning: Transcription occurs in the nucleus; translation occurs in the cytoplasm. Codons are on mRNA; anticodons are on tRNA. Uracil replaces thymine in RNA. The Central Dogma is unidirectional under normal conditions. Reverse transcription is an exception. Do not confuse transcription and translation.