Transcription, RNA Processing, and Translation: Study Notes

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Ch. 17 – Transcription, RNA Processing, and Translation

Introduction to Transcription

Transcription is the process by which RNA is synthesized from a DNA template. This is the first step in gene expression, where information in DNA is converted into a functional RNA molecule.

Gene: A unit of heredity that encodes a product (RNA or protein).
Transcription: The synthesis of RNA using DNA as a template.
Promoter: DNA sequence where RNA polymerase binds to initiate transcription.
Terminator: DNA sequence signaling the end of transcription.

Example: In bacteria, the promoter is recognized by sigma factors, while in eukaryotes, transcription factors are required for RNA polymerase binding.

Overview of Transcription

Genes are located on DNA, which has two strands: the coding strand and the template strand. RNA is synthesized by pairing RNA nucleotides with the DNA template strand, following base-pairing rules (A-U, G-C).

Coding strand: Has the same sequence as the RNA (except T is replaced by U).
Template strand: Used by RNA polymerase to synthesize RNA.

Example: If the DNA coding strand is 5'-ATGCGT-3', the RNA transcript will be 5'-AUGCGU-3'.

Steps of Transcription

Transcription consists of three main steps: initiation, elongation, and termination.

Initiation: RNA polymerase binds to the promoter region of DNA. In prokaryotes, sigma factors help RNA polymerase recognize promoters. In eukaryotes, transcription factors are required.
Elongation: RNA polymerase moves along the DNA template, synthesizing RNA in the 5' to 3' direction.
Termination: RNA polymerase stops transcription upon reaching a terminator sequence, releasing the RNA molecule.

	Prokaryotes	Eukaryotes
Initiation	Sigma factor + RNA polymerase	Transcription factors + RNA polymerase II
Termination	Hairpin loop or Rho-dependent	Polyadenylation signal, followed by RNA processing

Eukaryotic RNA Processing & Splicing

In eukaryotes, the primary RNA transcript (pre-mRNA) undergoes several modifications before becoming mature mRNA:

5' Capping: Addition of a modified guanine nucleotide to the 5' end.
3' Polyadenylation: Addition of a poly(A) tail to the 3' end.
Splicing: Removal of non-coding introns and joining of coding exons by the spliceosome.

Alternative splicing allows a single gene to code for multiple proteins by varying the combination of exons included in the final mRNA.

Types of RNA

Cells use several types of RNA, each with a specific function:

Messenger RNA (mRNA): Carries genetic information from DNA to ribosomes for protein synthesis.
Ribosomal RNA (rRNA): Forms the core of ribosome structure and catalyzes protein synthesis.
Transfer RNA (tRNA): Brings amino acids to the ribosome during translation, matching amino acids to codons in mRNA.

Introduction to Translation

Translation is the process by which proteins are synthesized from mRNA templates. Ribosomes read the mRNA sequence and assemble amino acids into polypeptides.

Ribosome: Molecular machine composed of rRNA and proteins; has large and small subunits.
tRNA: Contains an anticodon that pairs with mRNA codons and carries a specific amino acid.

	Prokaryotes	Eukaryotes
Ribosome Subunits	50S + 30S = 70S	60S + 40S = 80S

Ribosomal Binding Sites

Each ribosome has three binding sites for tRNA:

A site (Aminoacyl): Holds the tRNA carrying the next amino acid to be added.
P site (Peptidyl): Holds the tRNA with the growing polypeptide chain.
E site (Exit): Where discharged tRNAs leave the ribosome.

Steps of Translation

Translation occurs in three main steps:

Initiation: The small ribosomal subunit binds to mRNA and the initiator tRNA (carrying methionine) at the start codon (AUG). The large subunit then joins.
Elongation: Amino acids are added one by one to the growing polypeptide chain as the ribosome moves along the mRNA.
Termination: When a stop codon is reached, release factors bind, causing the polypeptide to be released and the ribosome to dissociate.

Post-Translational Modification

After translation, proteins may undergo further modifications, known as post-translational modifications (PTMs), which can affect their function, localization, or stability.

Common PTMs: Methylation, Acetylation, Hydroxylation, Phosphorylation, Glycosylation, Lipidation, Sulfation, Disulfide bond formation.

Example: Glycosylation is the addition of carbohydrate groups to proteins, often important for protein folding and stability.

Comparison of Transcription and Translation

	Transcription	Translation
Product Formed	RNA Molecule	Protein
Macromolecule Change?	No (Nucleic Acid to Nucleic Acid)	Yes (Nucleic Acid to Protein)
Major Enzyme/Structure	RNA Polymerase	Ribosome
Location	Nucleus (Eukaryotes)	Cytoplasm
Direction of Synthesis	5' to 3'	N-terminus to C-terminus

Genetic Code and Codon Table

The genetic code is a set of rules by which information encoded in mRNA is translated into proteins. Each codon (a sequence of three nucleotides) specifies a particular amino acid.

Start codon: AUG (Methionine)
Stop codons: UAA, UAG, UGA

Example: The mRNA sequence 5'-AUGGCCUAA-3' codes for the amino acids Met-Ala-Stop.

Key Equations and Concepts

Base pairing during transcription: DNA: A – T, G – C RNA: A – U, G – C
Direction of synthesis: Both DNA and RNA are synthesized in the 5' to 3' direction.

Summary Table: Transcription vs. Translation

Feature	Transcription	Translation
Template	DNA	mRNA
Product	RNA	Protein
Location (Eukaryotes)	Nucleus	Cytoplasm
Enzyme	RNA Polymerase	Ribosome

Practice Questions (Examples)

Which strand of DNA has the same sequence as the RNA molecule (except T/U)? Answer: Coding strand.
What is the function of the release factor during translation? Answer: It binds to the stop codon in the A site, causing the polypeptide to be released.
What is alternative splicing? Answer: The process by which different combinations of exons are joined to produce multiple mRNA variants from a single gene.

Additional info: These notes are based on the "Freeman - Biological Science" Ch. 17 materials and include both conceptual explanations and practice questions for exam preparation.