D.N.A.The universal code

Nucleotides – building blocks of DNA

• Nucleotide sub-units in DNA are assembled head-to-tail forming a chain.

• Each nucleotide has a sugar (deoxyribose) part, a phosphate part and an N-containing base.

• Four different kinds of N-containing bases are found in DNA: adenine(A), thymine(T), guanine(G) and cytosine(C).

Typical nucleotide

Phosphate

N-containing base

Deoxyribose sugar

P

A

When many nucleotides join to form a chain, a bond forms between the sugar of one of the nucleotide and the phosphate group of the next and so on. So, one chain runs from ‘head-to tail’. With a phosphate group at then head end (also known as the ‘5 end) and a sugar molecule at the tail end (also known as the ‘3 end).

P

A

P

A

‘5 end

‘3 end

History of DNA

• In 1953, James Watson (1928-) and Francis Crick (1916-2004) announced they had identified the 3D structure of DNA as being two nucleotide chains arranged to form a double helix.

James Watson

Francis Crick

Key features

• Each DNA molecule consists of two nucleotide chains.

• The chain runs in opposite directions and are said to be anti-parallel.

• The sugar-phosphate backbones are on the outside and they coil around each other to form a molecule with constant diameter.

• The pairing of bases is very specific. A with T and C with G. They are said to be complementary.

• Weak Hydrogen bonds form between base pairs.

Two groups of bases

• Purines: Adenine and Guanine

(two-ringed bases)

• Pyrimidines: Thymine, Cytosine and Uracil

(single -ringed bases)

A G

T C U

Chemical formulae for the complementary base pairs in DNA

Note: two hydrogen bonds between A-T pair and three between the G-C pair.

D.N.A. Structure‘5 end

‘3 end ‘5 end

‘3 end

What is a gene?

• A unit of heredity made up of a sequence of DNA.

• A sequence of DNA that has a function.• Information that is expressed as a

polypeptide/protein.• Alleles are alternate states (or forms of a gene).• A whole set of genes in an organism is a

genome.• Genes vary in size form about 100 to 2.5 million

base pairs.• Humans are thought to have 20,000 – 30,000

genes

DNA and genes are packaged into chromosomes

The length of DNA in one cell reaches about 1.8 m but it fits into a nucleus, which in a human cell is only 6μm wide.

This is because DNA is tightly packaged into chromosomes.

Chromosomes consist of DNA and therefore carry genes. They constantly change their appearance during the life of a cell and come in various shapes and sizes

A typical chromosome

• DNA is coiled around small proteins called histones.

• Where DNA is wrapped around a core of histone proteins it forms a particle called a nucleosome. This gives the DNA strand the appearance of a strand of beads.

• This arrangement of DNA wrapped around histones serves to package the DNA efficiently and to protect it from enzymatic degradation.

• When eukaryotes prepare to divide, the chromosomes become condensed.

• The nucleosomes fold themselves in a regular manner, producing supercoils.

• When they are highly condensed they can be seen under a light microscope.

Packaging of DNA

Histone proteins

Other ways DNA can be represented

Each gene has a position on a chromosome

• Each gene has a particular position, called a locus, on a specific chromosome. The within a gene there are coding regions referred to as exons and they are separated by non-coding regions called introns.

Introns are non-coding regions and exons are coding regions.

Does size matter?

The genome of the humble onion is six times larger that the human genome!

After sequencing of genomes began in 2001,it became apparent that a more

complex organism does not necessarily have more genes.

Common name Approx. size of genome (mbp)

Fruit fly 180

Human 3,100

Onion 18,000

Newt 84,000

Amoeba 670,000

How many chromosomes?

• All nucleated cells of an organism contain a fixed number of chromosomes. The number of chromosomes in somatic cells is characteristic of members of a species.

• The number of sets of chromosomes in a cell is called the ploidy level.

• Haploid (n) = one set

• Diploid (2n) = two sets

Diploid numbers of chromosomes in various species

Organisms Diploid number (2n)

Fruit fly 8

Koala 16

Human 46

Chimpanzee 48

Dingo 78

Garden pea 14

Fern 1260

Mould 2

The number of genes vary between species.

Common Name Number of genes

Human 30,000

Mustard plant 27,000

Fruit fly 14,000

Baker’s yeast 6,000

Should we be offended that a weedy mustard plant has almost as many genes as a human?

Synthesising new DNA

• DNA needs to be copied exactly so that it can be passed from one cell to another. This process is called DNA replication.

• During DNA replication each strand of parental DNA molecule acts as a template strand on which a new strand is synthesised.

DNA Replication

• DNA replication begins when a region of double-stranded DNA unwinds to form a region of single-stranded DNA.

• Nucleotide building blocks are the raw materials for the process of DNA replication.

• The nucleotides come into place – where there is a G in the template strand, a C-containing nucleotide is brought into place.

• In this way, the base sequence of the double-strand of DNA controls the order of the nucleotides in two new single strands of DNA.

• This process is catalysed by the enzyme DNA polymerase.

• Each nucleotide joins to its neighbour in the chain by a strong sugar-phosphate bond.

The strand is always copied in

the ‘5 –’3 direction.

Which model of DNA replication?

How do we know which model is correct?

The mystery of the model of DNA replication was solved in 1958 by Meselsohn and Stahl.

• They took bacterial cells and allowed them to multiply for several generations in a growth medium containing ‘heavy’ nitrogen (N-15) only, so that all nitrogen atoms in these bacterial DNA were heavy nitrogen.

• These bacterial cells were then transferred to a new growth medium containing only normal ‘light’ nitrogen (N-14).

• After one generation, a sample of the new bacterial cells was collected. Any new DNA made by this new generation could only include N-14 atoms.

• The DNA was extracted and the relative weight of the DNA was determined using a centrifuge.

• The lower down the tube, the heavier the DNA.

They obtained a single band with their experiment

showing that replicated DNA contained one

original band and one new chain. This

means that DNA replication is semi-

conservative