Chapter 17-From Gene to Protein

Chapter-17
From Gene to Protein

In 1909 the British physician Archibold Garrod postulated that the symptoms of an inherited disease reflect a persons inability to make a particular enzyme.

Example: Alkaptonuria- (black urine) In this disease your urine turns black due to the presence of the chemical alkapton.

Types of Proteins

Enzymes

Transport proteins

Hormones

Antibodies

Contractile proteins

Figures-17.2-17.3

Transcription- the synthesis of mRNA under the direction of DNA

Translation- the actual synthesis of a protein which occurs in the ribosomes under the direction of mRNA and the help of tRNA.

Chain of command is: DNA>mRNA>Protein

Codons

Each triplet (3) in the mRNA is called a codon and codes for 1 amino acid

Example: UGG = the amino acid Tryptophan

UUU = the amino acid Phenylalanine

The genetic code is based on the triplets in the mRNA (figs.17.3 and 17.4)

Codons

There are 64 codons that make up the genetic code.

Since there are only 20 amino acids it is obvious then that some codons will code for the same amino acid.

All life forms are based on the genetic code.

This is the basis for genetic engineering

Introns and Exons
Fig.17.9

Before mRNA leaves the nucleus it must be cleaned up

Introns- are noncoding segments of mRNA that must be removed before the mRNA gets to the ribosomes.

Exons- are the coding segments that remain in the functional mRNA

The role of tRNA in Protein Synthesis

tRNA- is called an anticodon. It brings to the ribosomes whatever amino acids the mRNA is asking for

Fig 17.11

Structure of Ribosomes

Ribosomes are the sites where actual assembly of proteins takes place

They are composed of 2 major parts

Large subunit (composed of an A site a P site and a E site)

Small subunit

Ribosome Structure

P site- holds the growing amino acid chain

A site- hold the tRNA carrying the next amino acid to be added to the growing amino acid chain

E site- discharges the tRNA from the ribosomes. Now they are free again to bring in new amino acids to the P site.

Antibiotics and Ribosomes

Certain drugs can paralyze prokaryotic (bacteria) ribosomes without hurting eukaryotic ribosomes

Examples: Tetracycline and streptomycin can interfere with protein synthesis in bacteria

How do new proteins know where to go?

Fig 17.19

Signal peptide

Signal recognition particle (SRP)

Receptor protein

Mutations

Mutation- a change in the genetic material (DNA) of a cell or a virus.

A mutation can be : * Chromosomal or *Point mutations- changes in the bases A, G, C or T

Example: Fig 17.21 and 17.22

Mutations are caused by

Spontaneous- due to errors in DNA replication or repair

Mutagens- chemical or physical agents that interact with DNA and change its sequence.

Physical- x- rays, UV light, etc.

Chemical- tar, nictonine, benzene, formaldyhyde

3 bases = 1 codon
1 codon = 1 amino acid

If a protein is 5 amino acids long how many bases (A,T,G,C) did it take to code for that protein________

If a protein was made from 6 codons then how many amino acids long is that protein_____

If a protein is made from 27 bases then how many amino acids long is it______