DNA

Nucleic Acids and Protein Synthesis

• All functions of a cell are directed from some central form of information.
• This "biological program" is called the Genetic Code. - The way cell store information regarding it's structure and function.

History

• For years the source of heredity was unknown. This was resolved after numerous studies and experimental research by the following researchers:

1. Fredrick Griffith

• He was studying effects of 2 strains of an infectious bacteria, the "smooth" strain was found to cause pneumonia & death in mice. The "rough" strain did not. He conducted the following experiment:

Bacteria Strain injected into mouse	Result
Smooth Strain	Mouse dies
Rough strain	Mouse Lives
Heat-Killed Smooth strain	Mouse lives
Rough Strain & Heat killed smooth strain	*MOUSE DIES*

• The last condition was unusual, as he predicted that the mouse should live
• Concluded that some unknown substance was Transforming the rough strain into the smooth one

---

1. Avery, McCarty & MacLeod

• Tried to determine the nature of this transforming agent. Eg. Was it protein or DNA?
• Degraded chromosomes with enzymes which destroyed proteins or DNA
• Samples with Proteins destroyed would still cause transformation in bacteria indicating genetic material was DNA

3. Hershey-Chase

• 1 virus was "tagged" with ³²P on it's DNA
• The other was "tagged" ³⁵S on it's protein coat.
• Researchers found the radioactive P in the bacteria, indicating it is DNA, not protein being injected into bacteria.

 

1. Watson & Crick

• The constituents of DNA had long been known. Structure of DNA, however was not.
• In 1953, Watson & Crick published findings based on X-ray analysis and other data that DNA was in the form of a "Double Helix".
• Their findings show us the basic structure of DNA which is as follows.

DNA Structure

• DNA is Formed of in a "Double Helix" - like a spiral staircase.

DNA Molecule-note "double helix" shape

• DNA is formed by Nucleotides
• These are made from 3 components
• A 5-Carbon Sugar
• A Nitrogenous base
• A Phosphate group

• For DNA There are 4 different Nucleotides categorized as either Purines or Pyramidines. These are usually represented by a letter. These Are:
• Adenine (A)
• Cytosine (C)
• Guanine (G)
• Thymine (T)

• Each "Rung" of the DNA "staircase" is formed by the linking of 2 Nucleotides through Hydrogen Bonds.
• These Hydrogen bonds form only between specific Nucleotides. This is known as Base Pairing. The rules are as follows:
• Adenine (A) will ONLY bond to Thymine (T) (by 2 hydrogen bonds)
• Cytosine (C) will ONLY bond to Guanine (G) (by 3 hydrogen bonds)

Central Dogma of genetics.

• Central Dogma holds that genetic information is expressed in a specific order. This order is as follows:

Central Dogma

• There are some apparent exceptions to this.
• Retroviruses (eg. HIV) are able to synthesize DNA from RNA

---

DNA Replication

• DNA has unique ability to make copies of itself
• This is a major "driving force" of living things.
• Does so through the process of DNA Replication.
• Complex process
• DNA "Unzips itself" forming two strands with an exposed Nucleotide.
• An nucleotide which forms the appropriate Base-pair bonds with the exposed nucleotide. This is facilitated by the enzyme DNA Polymerase.
• The process moves down the DNA molecule, and once complete, results in two identical DNA strands.
• Transcription proceeds continuously along the 5'® 3' direction (This is called the leading strand)
• Proceeds in fragments in the other direction (called the lagging strand) in the following way
• RNA primer attached to a segment of the strand by enzyme primase.
• Transcription now continues in the 5'® 3' direction forming an okazaki fragment. Until it reaches the next fragment.
• The two fragments are joined by DNA ligase

DNA Replication

---

RNA Transcription

• The cell does not directly use DNA to control the function of the cell.
• DNA is too precious and must be kept protected within the nucleus.
• The Cell makes a working "Photocopy" of itself to do the actual work of making proteins.
• This copy is called Ribonucleic Acid or RNA.
• RNA differs from DNA in several important ways.

1. It is much smaller
2. It is single-stranded
3. It does NOT contain Thymine, but rather a new nucleotide called Uracil which will bind to Adenine.

• RNA is produced through a process called RNA Transcription.
• Similar to DNA Replication.
• Small area of DNA "Unzips" exposing Nucleotides
• This area is acted on by an enzyme called RNA Polymerase, which binds nucleotides (using uracil) to their complimentary base pair.
• This releases a long strand of Messenger RNA (mRNA) which is an important component of protein synthesis.

RNA Transcription

---

Protein Synthesis & The Genetic Code

• The Sequence of nucleotides in an mRNA strand determine the sequence of amino acids in a protein
• Process requires mRNA, tRNA & ribosomes.

mRNA

• Each three Nucleotide sequence in an mRNA strand is called a "Codon" Each Codon codes for a particular amino acid.
• The codon sequence codes for an amino acid using specific rules. These specific codon/amino acid pairings is called the Genetic Code.

---

tRNA

• There is a special form of RNA called Transfer RNA or tRNA.

tRNA

• Each tRNA has a 3 Nucleotide sequence on one end which is known as the "Anitcodon"
• This Anticodon sequence is complimentary to the Codon sequence found on the strand of mRNA
• Each tRNA can bind specifically with a particular amino acid.

---

Ribosome

• Consists of two subunits
• Large subunit
• Small subunit
• Serves as a template or "work station" where protein synthesis can occur.

---

Protein Synthesis

• Protein synthesis is a complex, many step process, it is as follows.
• An mRNA strand binds to the large & small subunits of a ribosome in the cytoplasm of the cell
• This occurs at the AUG (initiation) codon of the strand.
• A tRNA molecule with an attached amino acid binds to the mRNA strand.
• Note: This occurs with complimentary codons & anti-codons.
• Another tRNA binds to the adjacent codon of the mRNA
• A peptide bond is formed between the amino acids
• The first tRNA is released, and another tRNA binds next to the second, another peptide bond is formed.
• This process continues until a stop codon is reached.
• The completed polypeptide is then released.

Eukaryotic DNA

• DNA in the eukaryotic cell is packaged in the form of chromatin. Which forms a chromosome.
• Chromatin is a complex of DNA and histone proteins
• DNA is wrapped around a core of 8 histone protein molecules, and linked to a H1 histone protein.
• Each of this units is called a nucleosome

This continuing chain of nucleosomes is attached to a protein scaffold.
This scaffold loops around itself, eventually forming a chromosome.