Transcription and Translation Lesson: Enzymes, Codons, and RNA Decoding

Lesson Overview

• The Role of DNA in Protein Production
• What is Transcription?
• RNA Polymerase and Direction of Synthesis
• Structure and Properties of mRNA
• Understanding Base Pairing in DNA
• Transcription Initiation and Enzyme Structure
• What is Translation?
• Process of Translation
• Codons and the Genetic Code
• Comparing Transcription and Translation

Living organisms are made of cells, and these cells carry out countless functions to keep the body working. Many of these functions depend on proteins, and the instructions for making proteins are found in DNA. Two key processes-transcription and translation-help convert DNA instructions into functional proteins.

The Role of DNA in Protein Production

DNA contains the full set of instructions for making every protein in the body. These instructions are divided into genes, which are segments of DNA that code for specific proteins or RNA molecules.

• A gene is a section of DNA that carries the code for making a protein or RNA.
• The entire set of genes in a cell is called the genome.
• Proteins are not made directly from DNA. First, a copy of the gene must be made.

What is Transcription?

Transcription is the first step in gene expression. It is the process of copying the genetic code from DNA into a molecule called messenger RNA (mRNA).

Where and How It Happens:

• Location: Nucleus of the cell
• Enzyme Involved: RNA Polymerase
• Purpose: To produce mRNA that carries genetic instructions from the DNA to the cytoplasm

Stages of Transcription:

1. Initiation: RNA polymerase binds to a DNA region called the promoter.
2. Elongation: RNA polymerase reads the DNA template and builds a single-stranded RNA molecule.
3. Termination: Transcription ends when RNA polymerase reaches a termination sequence.

RNA Polymerase and Direction of Synthesis

RNA polymerase reads the DNA strand in the 3' to 5' direction and builds RNA in the 5' to 3' direction.

• This directionality ensures proper base pairing.
• The rules for RNA base pairing:
  • Adenine (A) pairs with Uracil (U)
  • Thymine (T) pairs with Adenine (A)
  • Cytosine (C) pairs with Guanine (G)
  • Guanine (G) pairs with Cytosine (C)

Structure and Properties of mRNA

Messenger RNA (mRNA) is the product of transcription. It carries the genetic message from DNA to the ribosome for protein synthesis.

• mRNA is short-lived and degrades after use.
• It contains codons, which are groups of three bases that code for amino acids.

Understanding Base Pairing in DNA

DNA is a double-stranded molecule. Its strands are held together by hydrogen bonds between complementary base pairs:

• Adenine (A) - Thymine (T)
• Guanine (G) - Cytosine (C)

Hydrogen bonds are weak enough to break during transcription, allowing the DNA strands to separate and serve as templates.

Transcription Initiation and Enzyme Structure

To study how transcription begins and how RNA polymerase functions, scientists used a technique called X-ray Crystallography. This method helps determine the 3D structure of molecules.

What is Translation?

Translation is the second step of gene expression. It converts the mRNA sequence into a sequence of amino acids, forming a protein.

Where It Happens:

• Location: Cytoplasm, at the ribosome

Molecules Involved:

• mRNA: Contains codons (genetic code)
• tRNA: Transfers specific amino acids based on codons
• Ribosome: Builds the protein by joining amino acids

Process of Translation

Initiation:

• Ribosome binds to mRNA.
• Translation starts at the start codon (AUG), which codes for methionine.

Elongation:

• tRNA brings amino acids to the ribosome.
• Each tRNA has an anticodon that matches an mRNA codon.
• The ribosome forms peptide bonds between amino acids.

Termination:

• Process ends at a stop codon (UAA, UAG, UGA).
• The completed protein is released.

Codons and the Genetic Code

The genetic code uses codons, sequences of 3 RNA bases that correspond to specific amino acids.

There are 64 codons but only 20 amino acids, which means several codons can code for the same amino acid-a feature known as redundancy.

Comparing Transcription and Translation

Transcription and translation are essential molecular processes that transform genetic information stored in DNA into functional proteins. Through transcription, DNA sequences are transcribed into mRNA, which then guides the synthesis of proteins during translation. Understanding the roles of enzymes like RNA polymerase, the structure of mRNA, and the function of codons and ribosomes provides a clear framework for grasping gene expression.