genes and rna

I. RNA Overview

General Features of RNA

  • Made of nucleotides, each containing:

    • Ribose sugar (5-carbon)

    • Phosphate group

    • Nitrogenous base

  • RNA bases:

    • Adenine (A)

    • Cytosine (C)

    • Guanine (G)

    • Uracil (U) instead of thymine

Key Differences: RNA vs DNA

Feature

DNA

RNA

Strands

Double

Single

Sugar

Deoxyribose

Ribose

Base

Thymine

Uracil

🔗 Cause → Effect
Single-stranded RNA → flexible → can fold into functional shapes

II. Three Types of RNA

mRNA (Messenger RNA)

  • Carries genetic information from DNA to ribosome

  • Synthesized during transcription

  • Read in codons (3-base sequences)

rRNA (Ribosomal RNA)

  • Structural and functional component of ribosomes

  • Catalyzes peptide bond formation

tRNA (Transfer RNA)

  • Transfers specific amino acids to ribosome

  • Contains:

    • Anticodon complementary to mRNA codon

    • Amino acid attachment site

🔗 Cause → Effect
Correct codon–anticodon pairing → correct amino acid sequence

III. Transcription

Definition

  • Process where RNA polymerase synthesizes mRNA from DNA template

Key Features

  • Occurs in:

    • Cytoplasm (prokaryotes)

    • Nucleus (eukaryotes)

  • mRNA sequence is complementary to DNA template strand

IV. Prokaryotes vs Eukaryotes (mRNA Processing)

Prokaryotes

  • DNA mostly coding

  • No introns

  • Transcription and translation occur simultaneously

  • mRNA has short lifespan

🔗 Cause → Effect
No nuclear membrane → translation begins before transcription ends

Eukaryotes

  • DNA mostly noncoding

  • Genes contain:

    • Introns (noncoding)

    • Exons (coding)

  • mRNA must be modified before translation

mRNA Modification Steps

  1. Introns removed

  2. Exons spliced together

  3. Mature mRNA exits nucleus

🔗 Cause → Effect
Splicing removes nonsense sequences → functional protein produced

V. Translation (Protein Synthesis)

Definition

  • Process of decoding mRNA into a polypeptide

  • Occurs in the cytoplasm at ribosomes

Four Steps of Translation

1. Activation

  • Amino acids bonded to tRNA

  • Produces charged tRNA

2. Initiation

  • Small ribosomal subunit binds to 5' end of mRNA

  • Start codon (AUG) recognized

  • Large subunit attaches

3. Elongation

  • Amino acids added one at a time

  • Peptide bonds form

  • Assisted by elongation factors

4. Termination

  • Stop codon reached (UAA, UAG, UGA)

  • Release factor causes ribosome to detach

🔗 Cause → Effect
Stop codon → release factor binding → polypeptide released

VI. Role of Ribosomes

  • Composed of rRNA + proteins

  • Align mRNA and tRNA

  • Catalyze peptide bond formation

  • Ensure correct translation frame

VII. One Gene–One Enzyme Hypothesis

Beadle & Tatum (1941)

  • Studied Neurospora crassa

  • Showed mutations affect enzyme production

  • Conclusion:

    • One gene codes for one enzyme (protein)

🔗 Cause → Effect
Gene mutation → defective enzyme → altered phenotype