RNAProtein Synthesis

Mutations

RNA differs from DNA in three ways:• an RNA molecule consists of a single-strand

of nucleotides• RNA has ribose (5-C sugar) rather than

deoxyribose• RNA has the pyrimidine uracil instead of

thymine

Function

mRNA

transmits information from

DNA and serves as a template for

protein synthesis

tRNAbrings amino acids

to ribosomes for protein synthesis

rRNA rRNA and proteins make up ribosomes

Has 2 stages:Transcription and Translation

Transcription is the first part of protein synthesis.

* What does it mean to transcribe? Step 1: An enzyme called RNA

polymerase attaches to DNA and “unzips” it.

Step 2: RNA polymerase then uses one strand of DNA as a template from which nucleotides are assembled into a strand of mRNA.

RNADNA

RNApolymerase

Adenine (DNA and RNA)

Cystosine (DNA and RNA)

Guanine(DNA and RNA)

Thymine (DNA only)

Uracil (RNA only)

The nucleotide sequence of DNA is converted into an RNA nucleotide sequence.

Let’s say one strand of a DNA molecule had this sequence

AGCCTACGTAAG What would the mRNA sequence

produced be?

DNA: AGCCTACGTAAG

mRNA: UCGGAUGCAUUC

Proteins are made by joining amino acids together into a long chain. This chain is a protein.

The functions and properties of a protein are determined by the order in which 20 different types of amino acids are joined.

The “language” of mRNA is called the genetic code.

RNA has 4 bases: adenine, uracil, cytosine, and guanine.

The “language” of RNA is written in only 4 letters – AUGC

The code is read three letters at a time

Each “word” is 3 letters long Each “word” of 3 nucleotides is called

a codon

The 4 different bases can be arranged into 64 possible codons (4X4X4=64), but make only 20 different amino acids.

This is because some amino acids correspond to several different codons.

Example:

Practice:a) For what amino acid

does the codon CGG code?

b) For what amino acid does the codon AUG code?

c) For what amino acid does the codon UGA code?

UAA UAG UGA

AUGStart Codon:

Stop Codons:

The 2nd stage of protein synthesis is translation.

*What does it mean to translate? The sequence of nucleotides in mRNA

serves as instructions for the order in which amino acids should be joined together in a protein. Think of mRNA as the “recipe” for a protein.

Transcription occurs in the nucleus. In translation, the mRNA travels out

of the nucleus, and moves through the cytoplasm and attaches to a ribosome. So, translation occurs at the ribosomes.

Steps of translation:1. mRNA attaches to a ribosome2. As each codon of the mRNA molecule

moves through the ribosome, the proper amino acid is brought to the ribosome by tRNA molecules.

3. Each tRNA molecule has 3 unpaired bases on it. These bases are called anticodons.

4. The amino acids are strung together like a chain (remember, a protein is a polymer) by the ribosome, forming a long protein molecule.

5. The tRNA’s are released from the amino acids.

6. The protein chain grows longer and longer until the ribosome reaches a stop codon on the mRNA molecule. Then the protein is released from the ribosome.

TAC AAA TTT CAG AAC ACC TTC CAT ATT

AUG UUU AAA GUC UUG UGG AAG GUA UAA

UAC AAA UUU CAG AAC ACC UUC CAU AUU

met phe lys val leu try lys val stop

DNA:

mRNA:

tRNA:

Poly-peptide:

•Gene Mutation – produce changes in a single gene•Chromosomal Mutations – produce changes in an entire chromosome

A gene mutation is a change in a DNA sequence.

Gene mutations that involve changes in only one or just a few nucleotides are called point mutations b/c they occur at a single point in the DNA sequence.

There are 3 types of point mutations: substitutions, deletions, and insertions.

Substitution Mutations One base is changed to another Usually affect no more than a single

amino acid, but could still have an effect on a protein.

Insertion Mutations One extra base is inserted into the

DNA sequence.

Deletion Mutations One base is left out of the DNA

sequence.

Since the genetic code is read 3 letters at a time, the message is shifted for every codon that follows. Hence, insertions result in frameshift mutations. They have a dramatic effect on the amino acid sequence, and thus, the protein.

* Question…during what event do these mutations usually occur?

Substitution InsertionDeletion

Chromosomal mutations involve changes in the number or structure of chromosomes.

We discussed many of these last chapter. Examples are Down syndrome, Turner syndrome, Fragile-X syndrome, Prader-Willi syndrome, and Klinefelter’s syndrome.

Deletion

Duplication

Inversion

Translocation