The Genetic Code by Loretta Ashu, David Owiredu and Ashley Wyche

1
The Genetic Codeby Loretta Ashu, David Owiredu
and Ashley Wyche
2
The Genetic Code
3

• Central Dogma
• DNA? mRNA? Protein
• Nucleotides are the building blocks that cells
  use to make their information storage and
  transfer molecules.
• Amino acids are the units that are put together
  to make protein.
• The function of the protein is determined by the
  order of the amino acid

4
How do nucleotides in mRNA molecules specify the
amino acid sequence in protein?

• They use 4 different nucleotides adenine,
  cytosine, guanine, and uracil. These 4 different
  molecules make 20 different amino acids that are
  used to synthesis proteins.
• A 3 letter code generates 64 possible codons.
• 4364 not 414 or 4216
• This makes ribosome to use a triplet code to
  translate mRNA into amino acid

5
Characteristics of genetic code

• The code is a triplet code
• The code is continuous
• The code is nonoverlapping
• The code is almost universal
• The code has start and stop signals
• The code is degenerate

6
Open Reading Frames

• Strings of codons not interrupted by a stop codon
  in the same reading frame.
• Accurate translation can only occur when
  ribosomes examine codons in the reading frame
  that is established by a genes start codon.
• Can be found after transcription with a segment
  of mRNA that begins with a start codon and ends
  with a stop codon
• In order for RNA to be translated there has to
  be an ORF, which codes for one protein.

7
Open Reading Frames

• RNA polymerase reads the template strand from 3
  to 5 find AUG.
• Coding strand (opposite strand) reads 5 to 3
  
• Start codons AUG
• Stop codons UGA, UAA, UAG

8
Open Reading Frames

• HOW MANY ORF?
• 3 AGUAAAAUGUUUACAUGCCCCUGA 5
• 5 UCAUUUUACAAAUGUACGGGGACU 3
• 2 ORF!!!!!

9
mRNA Modification

• DNA genes are transcribed to mRNA by RNA
  polymerase.
• Thymine is replaced by Uracil
• In eukaryotes mRNA is modified through a process
  called splicing before undergoing translation

10
What is splicing?

• The process of excising internal sequences of
  mRNA transcripts called introns, and rejoining
  the exons flanked by them
• Occurs in the nucleus of eukaryotes
• Spliceosomes enzyme activities
• Alternative splicing- the production of two or
  more mRNA molecules by using different splicing
  junctions.

11
Introns

• A nucleotide sequence in eukaryotes that must be
  excised from a structural gene transcript.
• Contains sequences that cannot be translated
  into the amino acid sequence of a polypeptide.
• Failure to slice results in premature stop codons
  or frame shifts
• GT-AG Rule

12
Exons

• The part of an mRNA molecule that specifies the
  amino acid sequence of a polypeptide during
  translation.

13
(No Transcript)
14
Cystic Fibrosis Gene

• 24 introns
• Over 1 million nucleotide long
• Causes production of excessive thick mucus in the
  lungs of individuals.