Notes: Chapter 13: RNA & Protein Synthesis

• 2 steps:– Transcription (DNA

is made into mRNA in the nucleus)

– Translation (mRNA is made into proteins by ribosomes in

the cytoplasm or ER)

RNA Structure

• Similar to DNA, but with some important differences:– Single strand (DNA double)– Ribose is the sugar (not deoxyribose like

DNA)– Uracil matches adenine (instead of

thymine like DNA)– Shorter than DNA

RNA Functions

• Three different types of RNA:

–mRNA (messenger) used as template to

make proteins– rRNA (ribosomal)

makes up ribosomes– tRNA (transfer)

matches amino acids to mRNA to help make proteins

Transcription• transcribe (to copy)• happens in the nucleus• DNA’s code is copied

onto RNA– 1. RNA polymerase binds

to beginning of a gene (promoter) and unwinds DNA

– 2. Complementary bases copied from 1 side of DNA

– 3. Termination (end) signal reached, mRNA and DNA free 

• http://www.youtube.com/watch?v=ztPkv7wc3yU

Translation

• RNA’s “language” is translated into amino acids (which will become a protein)

• RNA’s language is set of three nucleotides called a codon…3 nitrogen bases in a row

• Codons match with specific amino acids to make polypeptide chain (which will be modified to make a protein)

• 20 amino acids all together• More than 20 codons, so some amino

acids have more than one codon. There is also 1 start codon and 3 stop codons.

Translation

Translation1. ribosome attaches to mRNA @ start codon2. tRNA with amino acid matches mRNA codon

(area on tRNA that matches called an anticodon). This process is called initiation. 2 tRNAs can fit at one time.

3. ribosome moves down and matches next codon. 4. Amino acids form peptide bond and protein

continues to grow, 1 amino acid at a time. This process is called elongation.

5. ribosome reaches stop codon, mRNA, tRNAs, protein and ribosome released. This process is called termination.

The product is called a polypeptide. Modified in ER or Golgi to make a protein.

Translation

• Each mRNA can be used more than once, by more than 1 ribosome at a time, so many proteins can be translated from 1 transcribed piece mRNA.

• http://www.youtube.com/watch?v=-zb6r1MMTkc

Gene expression• The whole

process of going from DNA to RNA to a protein that runs a cell process is called gene

expression

Gene expression• After the polypeptide

chain is made, it is often modified in the ER or

Golgi Apparatus• The finished product

is a protein and it has a three dimensional shape that decides its

function

Mutations• Certain things can cause changes to the

DNA/RNA code. They are called mutagens and cause mutations.

• e.g. UV light, smoking, X rays, exposure to chemicals

• Not all are bad…some allow for adaptation and evolution

Point mutations• Occur to one base or a small number

of bases on DNA or RNA• Most are harmless, as many codons

often code for the same amino acid.• 3 kinds:– Substitution: one base is used instead of

another– Insertion*: one base is added– Deletion*: one base is left out– * lead to a frameshift mutation, where

everything is “off by one” base.

Point mutations

Chromosomal mutations

• Change to a gene, series of genes or an entire chromosome

• These are often helpful in plants, e.g. polyploidy…chromosomes do not separate during meiosis. Leads to seedless fruit that are bigger

• In animals, they are often harmful or even lethal. You might be

researching one disorder in the next unit on genetics…

Chromosomal mutations

• Deletion: one or more genes is/are left out when copied

• Duplication: one or more genes is/are copied more than once

• Inversion: one or more genes is/are copied backwards

• Translocation: one or more genes is/are moved from 1 chromosome to another

• Nondisjunction (not shown): homologs fail to separate during meiosis

Transcription