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Nucleic Acids and Protein Synthesis

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

• 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:

• 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

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

• 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.

• 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

• 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.

• 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

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

• 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.

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

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

• 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.

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Mr. Stanley

Last updated: March 11, 2004