Molecular Genetics - DNA, RNA, and Gene Action

Genetic Material, Gene Action, and Nucleic Acids

Introduction

  • This lecture introduces students to genetic material in humans, focusing on chromosomes, DNA, RNA, and genes.
  • It also explains gene action from DNA to RNA to protein.

Lecture Objectives

  • Describe the structure of DNA.
  • Explain DNA replication and list the major enzymes involved.
  • Differentiate between DNA and RNA structures.
  • State the roles of RNA in a cell.
  • Understand the structure and function of the genetic code.
  • Describe transcription.
  • Summarize translation.

Human Genome

  • The human genome is contained in every cell.
  • It consists of 23 chromosome pairs and a small mitochondrial DNA.

Chromosomes

  • A chromosome is a structure in the nucleus.
  • It carries genetic information in the form of genes.
  • It is passed from parents to offspring.
  • It consists of DNA tightly coiled around proteins, including histones and non-histone proteins.
  • Genes are arranged in linear order along the DNA.

DNA Structure

  • DNA (Deoxyribonucleic acid) consists of two chains of repeating nucleotides.
  • A nucleotide consists of:
    • Phosphate group
    • Five-carbon sugar: Deoxyribose sugar
    • Nitrogenous bases: Adenine (A), Guanine (G), Cytosine (C), and Thymine (T)
  • Nucleotides in a DNA strand are held together by phosphodiester bonds.

Characters of DNA

  • Double Helix: Two strands are wound together.
  • Complementary Base Pairing:
    • Adenine (A) pairs with Thymine (T).
    • Guanine (G) pairs with Cytosine (C).
    • The two strands are held together by hydrogen bonds.
  • Antiparallel: The strands run in opposite directions.

Gene

  • A gene is a segment of DNA that contains the code to make RNA or a polypeptide (protein).
  • Genes determine traits like eye or hair color.
  • The human cell nucleus contains about 30,000 genes located on 46 chromosomes.
  • A gene consists of:
    • Promoter (start) sequence
    • Exons (protein-coding regions)
    • Introns (non-protein-coding regions)
    • Terminator (stop) sequence

DNA Replication

  • DNA replication is the process of copying a DNA molecule.
  • During replication:
    1. The parental strands unwind.
    2. Each parental strand serves as a template for DNA polymerase to bind complementary bases: A with T, and G with C.

Enzymes Involved in DNA Replication

EnzymeFunction
HelicaseUnwinds the parental double helix.
Binding proteinsStabilize separated strands.
PrimaseAdds short RNA primer to the template strand.
DNA polymerase1. Binds nucleotides to form new strands. 2. Proofreading activity checks and replaces incorrect bases. 3. Removing RNA primer
LigaseSeals nicks in the sugar-phosphate backbone.

Differences Between DNA and RNA

FeatureRNADNA
StructureSingle-strandedDouble-stranded
BasesA, G, C, and Uracil (U)A, G, C, and Thymine (T)
SugarRiboseDeoxyribose
FunctionTransfers genetic codes from the nucleus to ribosomes to make proteins.Stores and transfers genetic information to daughter cells.
StabilityTransientPersists

Types of RNA and Their Function

  1. Coding RNA:
    • Messenger RNA (mRNA): Protein-coding RNA, carries genetic codes for protein synthesis.
  2. Non-coding RNAs (ncRNA):
    • Ribosomal RNA (rRNA): Part of the ribosome, participates in protein synthesis.
    • Transfer RNA (tRNA): Transfers specific amino acids to the mRNA/ribosome complex during protein synthesis.
    • Small nuclear RNA (snRNA): Forms part of the spliceosome, which removes introns from the gene.
    • Micro RNA (miRNA): Small RNA involved in the regulation of protein-coding genes.

Transcription (RNA Synthesis)

  • Involves the synthesis of RNA from a DNA template using RNA polymerase in the nucleus.
  • RNA polymerase unwinds the DNA double helix locally.
  • RNA polymerase then adds nucleotides to a growing chain in the sequence: A - U (Uracil in place of Thymine) and G - C.
  • RNA is transcribed from one strand, the template strand of DNA.

mRNA Processing in Eukaryotes

  • Transcription produces pre-mRNA that is processed into mature mRNA by the addition of:
    1. Cap (7-Methyl guanosine) to the 5' end.
    2. Poly (A) tail to the 3' end.
      • Facilitates the transport of mRNA from the nucleus to the cytoplasm.
      • Protects mRNA from degradation by hydrolytic enzymes.
    3. Introns (non-coding sequences) are cut out, and exons (protein-coding sequences) are reattached.

Genetic Code

  • The linear sequence of 3 nucleotides in mRNA that codes for a sequence of amino acids in a polypeptide chain during translation at the ribosome.
  • Features:
    1. Triplet: The genetic code is a triplet (e.g., AAA codes for Lysine).
    2. Has a start codon (AUG) and 3 stop codons (UAA, UAG, UGA).
    3. More than one codon may encode a single amino acid.

Translation (Protein Synthesis)

  • Translation is the process of protein synthesis in the cytoplasm of eukaryotic cells.
  • During translation, the sequence of codons in the mRNA at a ribosome directs the sequence of amino acids in a polypeptide.
  • The gene action from DNA to RNA to protein.

Steps of Translation

  1. Initiation:
    • All translation components bind together.
    • mRNA with start codon (AUG)
    • small ribosomal subunit
    • initiator tRNA with anticodon UAC
    • large ribosomal subunit binds to them
  2. Elongation:
    • tRNAs transfer amino acids to the ribosome.
    • tRNAs anticodons base-pair with mRNA codons.
    • Amino acids are joined by peptide bonds to form a new polypeptide chain.
  3. Termination:
    • mRNA stop codon appears in the ribosome.
    • The polypeptide chain and mRNA detach from the ribosome.

Summary

  • The human genome consists of 46 chromosomes arranged in 23 pairs located in the nucleus of each cell.
  • Chromosomes consist of genes made of DNA.
  • DNA is the genetic material.
  • DNA encodes information for protein synthesis.
  • DNA is replicated, passing on its information.