Protein Synthesis and Mutations
Differences Between DNA and RNA
Type of Sugar
A T G C
A U G C
Nucleus and Cytoplasm
Connection Between DNA and RNA in Protein Synthesis
RNA is responsible for protein synthesis and it needs DNA in order to complete the transcription step of protein synthesis.
Stages of Transcription
RNA polymerase binds to DNA at a specific sequence of nucleotides called the promoter.
The promoter contains an initiation site where transcription of the gene begins.
RNA polymerase unwinds DNA.
One of the unwound DNA strands acts as a template for the RNA synthesis.
RNA must be synthesized in the 5' to 3' direction.
Free ribonucleotides triphosphates from the cytoplasm are paired up with their complementary base on the DNA template.
RNA polymerase joins the ribonucleoside triphosphates to form an mRNA strand.
The DNA that has been transcribed, re-winds to it's original shape.
RNA polymerase continues to elongate until it reaches the terminator, a specific sequence of nucleotides that signals the end of transcription.
Transcription stops and mRNA polymerase and the new mRNA transcript are released from DNA.
The DNA double helix reforms.
The termination sequence usually consists of a series of adjacent adenines preceded by a nucleotide palindrome.
This gives an RNA molecule that assumes a stem-and loop configuration.
This stops RNA polymerase from transcribing any further.
Splicing- newly formed, "pre-messenger RNA" contains introns, which are removed by spliceosomes to produce a functional RNA molecule (containing only coding regions, or exons), which is ready to leave the nucleus.
Introns vs Exons
Nucleotide sequences seen within the genes which are removed through RNA splicing for generating a mature RNA molecule
Nucleic acid sequence which is represented in the RNA molecule
Not at all implicated with the protein coding
Codes of proteins
DNA bases that are found in between exons
DNA bases which are translated into mRNA
Stages of Translation
1. Initiation: When a small ribosome subunit finds the start sequence AUG, the codon (triplet) for the amino acid methionine, the large subunit joins the small one to form a complete ribosome and the protein synthesis is initiated.
2. Elongation: A new tRNA+amino acid enters the ribosome, at the next codon downstream of the AUG codon. If its anticodon matches the mRNA codon it’s base pairs and the ribosome link the two amino acids together. The ribosome then moves one triplet forward and a new tRNA+amino acid can enter the ribosome and it is repeated.
3. Termination: When the ribosome reaches one of three stop codons there are no corresponding tRNAs to that sequence. Instead termination proteins bind to the ribosome and stimulate the release of the polypeptide chain (the protein), and the ribosome dissociates from the mRNA.
Reasoning That Led to the Discovery of the Triplet Base Code
When you do the math, the most logical conclusion is the triplet base code because you can rule out having 1,2, and 4 base codes. Crick’s Experiment was used to find this conclusion.
Process of Making a Protein from DNA
Mutations that Can Occur in DNA
Substitution- two amino acids switch; produces a polypeptide chain with the wrong amino acid
Insertion- an extra amino acid is added; creates a large scale frame shift resulting in a completely new sequence of amino acids. The resulting protein is unlikely to have any biological activity.
Deletion- an amino acid is deleted; creates a large scale frame shift resulting in a completely new sequence of amino acids. The resulting protein is unlikely to have any biological activity.
Consequences on DNA Mutations on the Final Protein Product
- Causes an amino acid to change (Missense)
- Early Termination because of an early stop codon (Nonsense)
- Frameshift by insertion or deletion, causing everything to be thrown off
- Codes for the right amino acid (Silent)