THE CHEMICAL NATURE OF THE GENE

CREATED BY:

Aranda, CeciliaRosana, Jorge Leonard

The must be able to hold information and decode it (translate it) into an organism as it grows and develops

It must be able to copy itself so that it can be passed on to future generations

It must be a big molecule to hold the large amount of information required to build an organism

It must be a complex molecule to provide the necessary variation to code the instructions that control growth and development

NucleotidesCHONPPolynucleotides(Nucleic acids)

Amino acidsCHONSPolypeptides(proteins)

Fatty acids (and glycerol)

CHOLipids(Fats, oils and waxes)

MonosaccharidesCHOPolysaccharides(carbohydrates)

Building BlocksElementsBiological macromolecules

Tried to determine what genetic material was made of.

Pneumococcus bacteria on mice

2 STRAINS

S-typeSmooth colonies

Virulent

R-typeRough colonies

Avirulent

Innoculate into mice Innoculate into mice

Dead from pneumonia

Not killed

Live S-type found

Further test: Cultured lung fluid

No mice diedNo mice diedMice died from pneumonia

CONTROLHeat-killed S-type

only

CONTROLLive R-type only

EXPERIMENTLive R-type (harmless)

+Heat-killed S-type

Transformation of R-type to S-type Transformation was brought about by

some heat stable compound present in the dead S-type cells

Called the TRANSFORMING PRINCIPLE

Tried purifying the transforming principle to change R-type Pneumococcus to S-type

The compound that had the most effect was: Colourless, viscous and heat stable It contains phosphorus It was not affected by trypsin (a protease) or

amylase. It was inhibited by RNAase and DNAase

ConclusionThe transforming principle is a nucleic acid

DNA is the transforming principle and it is hereditary materialCriticismThe DNA was not totally pureIt was contaminated by a small amount of proteinThis protein could be the real transforming principleBUTWhen Avery and his team prepared purer extracts of DNA they became better at transforming the bacteria types

1. TranscriptionThe synthesis of mRNA uses the gene on the DNA molecule as a templateThis happens in the nucleus of eukaryotes

2. TranslationThe synthesis of a polypeptide chain using the genetic code on the mRNA molecule as its guide.

Found all over the cell (nucleus, mitochondria, chloroplasts, ribosomes and the soluble part of the cytoplasm).

Messenger RNA (mRNA) <5% Ribosomal RNA (rRNA) Up to 80% Transfer RNA (tRNA) About 15% In eukaryotes small nuclear

ribonucleoproteins (snRNP).

Single polynucleotide strand which may be looped or coiled (not a double helix)

Sugar Ribose (not deoxyribose) Bases used: Adenine, Guanine,

Cytosine and Uracil (not Thymine).

A long molecule 1 million Daltons Ephemeral Difficult to isolate mRNA provides the plan for the

polypeptide chain

Coiled Two subunits:

a long molecule 1 million Daltonsa short molecule 42 000 Daltons

Fairly stable Found in ribosomes Made as subunits in the nucleolus rRNA provides the platform for

protein synthesis

Short molecule about 25 000 Daltons Soluble At least 61 different forms each has a

specific anticodon as part of its structure. tRNA “translates” the message on

the mRNA into a polypeptide chain

Uses an enzyme RNA polymerase Proceeds in the same direction as replication

(5’ to 3’) Forms a complementary strand of mRNA It begins at a promotor site which signals

the beginning of gene is not much further down the molecule (about 20 to 30 nucleotides)

After the end of the gene is reached there is a terminator sequence that tells RNA polymerase to stop transcribing

NB Terminator sequence ≠ terminator codon.

In prokaryotes the transcribed mRNA goes straight to the ribosomes in the cytoplasm

In eukaryotes the freshly transcribed mRNA in the nucleus is about 5000 nucleotides long

When the same mRNA is used for translation at the ribosome it is only 1000 nucleotides long

The mRNA has been edited The parts which are kept for gene expression

are called EXONS (exons = expressed) The parts which are edited out (by snRNP

molecules) are called INTRONS.

Location: The ribosomes in the cytoplasm that provide the environment for translation

The genetic code is brought by the mRNA molecule.

The genetic code consists of the sequence of bases found along the mRNA molecule

There are only four letters to this code (A, G, C and U)

The code needs to be complex enough to represent 20 different amino acids used to build proteins.

If one base represented one amino acid this would only be able to produce

4 different combinations. (A, C, G and U) If pairs of bases represented each amino acid this

would only be able to produce 4 x 4 = 16 combinations. (AA, AC, AG, AU, CA, CC, CG,

CU etc) If triplets of bases represented each amino acid,

this would be able to produce 4 x 4 x 4 = 64 combinations

This is enough combinations to code for the 20 amino acids but is the code actually made of triplets?

Over 10 years biochemists synthesised bits of mRNA with different combinations

Then they used them to synthesise polypeptides

The results proved the logical answer was correct

The genetic code is made of triplets of bases called codons.

Proposed by Francis Crick 1958 DNA holds the coded hereditary information in

the nucleus This code is expressed at the ribosome during

protein synthesis in the cytoplasm The protein produced by the genetic

information is what is influenced by natural selection

If a protein is modified it cannot influence the gene that codes for it

Therefore there is one way flow of information:DNARNAProtein

DNA is a very stable molecule It is a good medium for storing genetic

material but… DNA can do nothing for itself It requires enzymes for replication It requires enzymes for gene expression The information in DNA is required to

synthesise enzymes (proteins) but enzymes are require to make DNA function

Which came first in the origin of life DNA or enzymes?

Certain forms of RNA have catalytic properties RIBOZYMES Ribosomes and snRNPs are ribozymes RNA could have been the first genetic

information synthesizing proteins… …and at the same time a biocatalyst Reverse transcriptase provides the possibility

of producing DNA copies from RNA