DNA as Genetic Material

Introduction

• Mendel was unaware of DNA
• It took time to determine genes are located on chromosomes
• There was initial debate on whether DNA or protein was the genetic material
• Viruses were used to establish DNA directs the formation of new viruses

Discovery of DNA as Genetic Material

• Alfred Hershey and Martha Chase determined DNA is the genetic material in 1952
• Experiment used E. coli-infecting viruses
• Experiments showed the virus injects DNA into the bacteria, not protein
• Radioactive tracers confirmed DNA enters the bacterium and directs new virus formation
• Thus, DNA serves as the genetic material

DNA Structure

• A race to determine DNA structure occurred
• DNA contains four types of nucleotides was already known
• DNA from different species was examined which led to Chargaff’s rules
• The amount of A, T, G, and C varies among species
• In each species, the amount of A equals T, and G equals C
• Nucleotides contain phosphate, a 5-carbon sugar, and a nitrogen-containing base
• Rosalind Franklin studied DNA structure using X-ray diffraction data, showing DNA has a helical structure with repeating sections
• In 1951, Watson and Crick combined data to build a DNA model
• The Watson-Crick model suggested how replication occurs
• The Watson-Crick model has held true and they won a Nobel Prize

DNA Double Helix Structure

• DNA has a double helix structure, like a twisted ladder
• Sides are formed by deoxyribose sugar and phosphate molecules bonded
• Rungs are formed of nitrogen bases
• Complementary base pairing occurs: A pairs with T, and G pairs with C
• Hydrogen bonds between bases hold the helix halves together

DNA Replication Overview

• DNA replication occurs before cell division
• The process copies DNA
• Two strands separate
• Each strand acts as a template for a new strand
• The result is a semiconservative new DNA molecule made of one parent strand and one new strand
• Replication requires
  • Unwinding via helicase
  • Complementary base pairing
  • Joining via DNA polymerase and DNA ligase
• The new DNA molecule is exactly identical to the original molecule

Semiconservative Replication Steps

• The parent strand unwinds and separates via helicase
• New strands form through complementary base pairing using DNA polymerase
• DNA ligase seals any breaks in the sugar-phosphate backbone
• A new DNA molecule results with one old strand and one new strand
• This molecule is identical to the original

Eukaryotic DNA Replication

• In eukaryotes, replication begins at numerous origins
• "Replication bubbles" form
• Bubbles spread bidirectionally until they meet

RNA Structure and Function

• RNA is ribonucleic acid
• RNA contains the sugar ribose
• RNA uses uracil (U) instead of thymine (T)
• RNA uses A, C, and G like DNA
• RNA is single-stranded
• There are three major types
  • Messenger RNA (mRNA)
  • Transfer RNA (tRNA)
  • Ribosomal RNA (rRNA)

DNA vs RNA comparison

• Similarities
  • Both are nucleic acids
  • Both are composed of nucleotides
  • Both have a sugar-phosphate backbone
  • Both have four different types of bases
• Differences
  • DNA is found in the nucleus; RNA is found in the nucleus and cytoplasm
  • DNA contains the genetic material; RNA is a helper to DNA
  • DNA sugar is deoxyribose; RNA sugar is ribose
  • DNA bases are A, T, C, and G; RNA bases are A, U, C, and G
  • DNA is double-stranded; RNA is single-stranded
  • DNA is transcribed (to give a variety of RNA molecules) and mRNA is translated (to make proteins)

The Three Types of RNA

• Messenger RNA (mRNA)
  • Produced in the nucleus using a DNA template
  • Carries genetic message to ribosomes
• Transfer RNA (tRNA)
  • Produced in the nucleus from DNA template
  • Transfers amino acids to ribosomes
  • Each type carries only one type of amino acid
• Ribosomal RNA (rRNA)
  • Produced in the nucleolus of the nucleus using a DNA template
  • Joins with proteins to form ribosomes
  • Ribosomes may be free / in polyribosomes (clusters) / attached to the ER

Gene Expression Concepts

• In the early 1900s, Sir Archibald Garrod suggested a link between inheritance and metabolic diseases, suggesting a link between genes and proteins
• DNA provides a blueprint to synthesize proteins
• Information flows from DNA to RNA to protein

Transcription and Translation

• In transcription, DNA serves as a template to make mRNA
• In translation, mRNA directs the sequence of amino acids in a protein
• rRNA and tRNA assist in translation

Transcription Detailed

• During it, complementary RNA is made from a DNA template
• A portion of DNA unwinds and unzips at the attachment point of RNA polymerase
• Bases join in the order dictated by the base sequence in the template DNA strand
• Newly made pre-mRNA must be processed

mRNA Processing

• Capping and addition of a poly-A tail provides stability
• Introns (non-coding regions) are removed
• Only exons (coding regions) are left
• Alternative splicing can produce different mRNA versions, leading to different proteins
• Mature mRNA now leaves the nucleus and associates with a ribosome in the cytoplasm

Importance of tRNA in Translation

• tRNA brings the amino acids
• Each has an anticodon , a group of 3 complementary bases to a mRNA codon
• When translation is complete, the protein's amino acid sequence comes from that originally specicified in DNA

Genetic Code

• Translates from nucleic acids to amino acids
• Triplet: 3 nucleotide sequence in DNA
• Codon: 3 nucleotide sequence in mRNA
  • A codon encodes a single amino acid
  • Start and stock codons

Ribosomes and Translation

• Ribosomes are composed of protein and rRNA and are the site of translation/protein synthesis
• Ribosomes bind mRNA and 2 tRNA molecules
  • P site: tRNA attached to a peptide
  • A site: site for newly arrived tRNA with an amino acid

Translation Phases

• Three main phases
  • Initiation
  • Elongation
  • Termination
• Initiation
  • mRNA binds to small subunit of ribosome
  • Large subunit then joins
• Elongation
  • Peptide lengthens one amino acid at a time
• Termination
  • One of three stop codons is reached
  • Release factor causes ribosomal subunits and mRNA to dissociate
  • Complete polypeptide is released