THE GENETIC MATERIAL

DNA Structure

DNA Replication

Transcription

Translation

Genetic Code

DNA It is made up of

chemical building blocks called nucleotides (phosphate group [phosphoric acid]; five-carbon sugar [deoxyribose]; and nitrogen base [one of the four types])

DNA Timeline 1865 - Gregor Mendel, working alone in an

Austrian monastery, discovers that some characteristics are inherited in ‘units’.

1868 - Friedrich Miescher isolated from the nuclei of pus cells an acidic substance rich in phosphorus which he called nuclein.

Early 1900’s - Biochemist Kossel identified the constituent nitrogenous bases of nucleic acid as well as 5-carbon sugar and phosphoric acid.

1928 - Frederick Griffith research showed that DNA is the genetic material

DNA Timeline 1929 – Phoebus Levene discovers that a sugar,

deoxyribose, is present in nucleic acids. 1937 - Development of DNA specific staining

technique by Feulgen and Rossenbeck. 1949 - Erwin Chargaff and his colleagues showed

that the base composition of DNA is related to the species of origin.

1951 - Rosalind Franklin takes her first X-ray diffraction pictures

Biologist James Watson and Physicist Francis Crick published the first description of the structure of DNA.

DNA STRUCTURE DNA molecule

consists of two strands, each of which is a polynucleotide chain

Each strand has a helical (spiral) shape, so that it has become known as the “double helix”

The polynucleotide chains run in opposite directions, meaning they are antiparallel.

This polynucleotide chains are joined by pairs of bases or known as complementary base pairing.

Complementation

The chain has a sugar-phosphate backbone with the organic base projecting outwards.

DNA REPLICATION

DNA replication is the process whereby an entire double-stranded DNA is copied to produce a second, identical DNA double helix.

The Process Replication begins on the

chromosome at a specific sequence of nucleotides called the origin.

Enzymes called helicases recognize this sequence and bind to this site.

This area where DNA separates and the bases are exposed is called the replication fork

Single-strand binding proteins, or SSBs, coat the single DNA strands to prevent them from snapping back together.

The primase enzyme uses the original DNA sequence as a template to synthesize a short RNA primer.

As replication moves from an origin along an unwound segment the addition and joining of bases on one strand can proceed continually in a 5' to 3' direction.

The short segments of newly assembled DNA are called Okazaki fragments.

The Okazaki fragments are joined together by ligase.

Enzymes involved in Replication

Helicases – separate the two DNA strands. Their action uses energy from ATP.

DNA binding proteins – keep the strands separate during replication.

DNA Polymerases – catalyze the polymerization of nucleotides to form a polynucleotide chain in the 5’ to 3’ direction. This allows one strand to be replicated continuously.

DNA ligase – an enzyme that joins the pieces of polynucleotide chain together.

DNA Replication is semi-conservative Semi-conservative

replication would produce two DNA molecules, each of which was composed of one-half of the parental DNA along with an entirely new complementary strand

In other words the new DNA would consist of one new and one old strand of DNA.

Experimental Evidence

In 1958, two American biochemists, Matthew Meselsohn and Franklin Stahl, conducted a neat experiment which gave strong support for the theory of semiconservative replication.

The experiment involved the growth of E. coli bacteria on a growth medium containing heavy nitrogen (Nitrogen-15 as opposed to the more common, but lighter molecular weight isotope, Nitrogen-14).

TRANSCRIPTION

Transcription is the synthesis of an RNA strand from a DNA template.

Transcription helps to magnify the amount of DNA by creating many copies of RNA that can act as the template for protein synthesis.

The RNA copy of the gene is called the mRNA.

Difference between DNA and RNA

DNA (Deoxyribonucleic Acid)

RNA (Ribonucleic Acid)

The pentose sugar is deoxyribose

The pentose sugar is ribose

Double stranded

The four types of base are: adenine, guanine, cytosine and thymine.

The four types of base are: adenine, guanine, cytosine and uracil.

Single stranded

Three Stages Initiation

– RNA polymerase binds to promoter, where transcription of the gene begins then unwinds DNA at the beginning of the gene.

Elongation– RNA polymerase add nucleotides to the 3' end of the strand – Free ribonucleotides triphosphates from the cytoplasm are

paired up with their complementary base on the exposed DNA template.

– RNA polymerase joins the ribonucleoside triphosphates to form an mRNA strand.

Termination– Transcription stops and mRNA polymerase and the new

mRNA transcript are released from DNA.

TRANSLATION

Translation is the process by which the nucleotide sequence of mRNA is converted to the amino acid sequence of a polypeptide.

It is a complex process which happens on ribosomes in the cytoplasm

Components of Translation

mRNA mRNA is the product of transcriptionmRNA is read in a series of triplets called

codons Each codon corresponds to one amino acid. E.g. AUG codes for the amino acid MET, AAG

codes for Lys, CAC codes for His and UAC codes for Tyr.

tRNATransfer RNA (tRNA)

is a single strand of 80 ribonucleotides.

It assumes a cloverleaf configuration

It functions as an interpreter between nucleic acid and peptide sequences by picking up amino acids and matching them to the proper codons in mRNA.

The Process Starting - Polypeptide synthesis is usually initiated

by the codon AUG. This is the codon for methionine so methionine is the first amino acid in the chain.

mRNA carries information in the form of codons which correspond to the amino acids to be used.

A ribosome moves along a strand of mRNA in the 5’ → 3’ direction and the codons are ‘read’ sequentially.

tRNA molecules bring amino acids to the ribosome, and these are added one by one to the growing polypeptide chain.

Continuation of the process:

The ribosome allows two molecules of tRNA to combine with the mRNA at any one time.

One tRNA molecule holds the growing polypeptide chain; the other carries the next amino acid to be added to the chain

Stopping - Synthesis ends when the ribosome reaches a ‘stop’ codon (UAA, UGA, or UAG) The mRNA, ribosome, and tRNA molecules separate, and the polypeptide chain is released.

THE GENETIC CODE

The genetic code is the set of rules by which information encoded in genetic material is translated into proteins by living cells.

Main FeaturesFeatures of code Comments

Triplet code of three nucleotides

Each of the 20 amino acids used to make proteins is represented by a three-letter abbreviation (a base triplet in DNA or a codon in mRNA)

Linear code reads from a starting point to a finishing point

The codon is always read in the 5’ → 3’ direction.

Degenerate code The code is degenerate (there are more codons than amino acids; most amino acids are coded for by more than one codon)

Punctuation codons The start and end of a coding sequence in a cistron is determined by specific codons: the ‘start’ signal is given by AUG, which codes for methionine; there are three ‘stop’ signals (UAA, UAG, and UGA).

Almost universal Most organisms share the same code; chloroplast and mitochondrial DNA have a slightly modified code; other exceptions to the universal genetic code are rare.

5’-ATGCCTAGGTACCTATGA-3’3’-TACGGATCCATGGATACT-5’

5’-AUGCCUAGGUACCUAUGA-3’

5’-AUG CCU AGG UAC CUA UGA-3’

N-MET-PRO-ARG-TYR-LEU-C

DNA

Transcription

decoded as

Translation

mRNA

Protein