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The Genetic Code

Essential Questions

1. Use the Genetic Code to identify condons, anti-codons and amino acids from a DNA sequence. (Essential Skill 2-2)

The Universal Genetic Code

• Codon = every 3 bases on mRNA that codes for a specific amino acid. – Start Codon = AUG =

Methionine

– Stop Codons = (UAA, UAG, UGA)

• This is what the tRNA reads

The Genetic Code is the same for all organisms!!!

tRNA – “reads” the codons

• Anticodon = 3 bases on tRNA that are complementary to the codon on mRNA.

• tRNA brings the correct amino acid to the ribosome/protein.

• The correct amino acid depends on the mRNA codons

How to Use the Genetic Code• Each codon has 3 letters

(nucleotides: G, C, A & U)– 1st letter can be found along

the left side of the chart– 2nd letter can be found along

the top of the chart– 3rd letter can be found along

the right side of the chart • it must be in the same row as

the first letter

• Where all three letters intersect is the name of the amino acid this codon codes for!– Each code always starts with

AUG (start) and ends with a stop codon!

• NOTE: The Genetic Code always uses the mRNA sequence!!!

tDNA TAC TCT GGG CTG ACT

mRNA

AA

Letter

Basic Decoding Example

tDNA TAC TCT GGG CTG ATC

mRNA AUG AGA CCC GAC UAG

AA Met Arg Pro Asp

Letter (start) W H S !

How Mutations Affect the Amino Acid Sequence

• A mutation (change in the nucleotide sequence) can affect protein structure in 3 ways:

– No Change: It doesn’t change the amino acid sequence

– Small Change: It changes only 1 amino acid

– Large Change: It changes the whole protein

Protein Synthesis Review

DNA mRNA protein

Transcription vs. Translation

• Transcription = DNA into mRNA (occurs in nucleus)

• Translation = mRNA into Protein (occurs at ribosome)

– tRNA carries the amino acids

– rRNA makes up the ribosomes

Protein Synthesis Review