DNA

• Need to know – molecular components, role of each.

• Revisit y12

Prokaryotes

• ‘simple’ cells, lack many organelles, DNA not bound in a membrane.

• Relatively small cells

• Cell wall of peptidoglycan

Eukaryotes

• Relatively large

• Membrane bound DNA

• Many organelles

• Fungi, protists, animals and plants

DNA• DNA is often called

the blueprint of life.• In simple terms,

DNA contains the instructions for making proteins within the cell.

Why do we study DNA?

We study DNA for many reasons, e.g.,

• its central importance to all life on Earth,

• medical benefits such as cures for diseases,

• better food crops.

The Shape of the Molecule

• DNA is a very long polymer.

• The basic shape is like a twisted ladder or zipper.

• This is called a double helix.

One Strand of DNA• The backbone of

the molecule is alternating phosphate and deoxyribose, a sugar, parts.

• The teeth are nitrogenous bases.

phosphate

deoxyribose

bases

One Strand of DNA

• One strand of DNA is a polymer of nucleotides.

• One strand of DNA has many millions of nucleotides.

nucleotide

Chromosomes

• The DNA is joined together into structures called chromosomes.

• The pinch in the middle is the centromere

• Stripes are the banding pattern

Genes• Each chromosome is divided up into areas called genes.

• Each gene codes for a single trait or characterisitic.

• Eg eye colour, height

• You get one copy of each gene from each parent.

Fig 2.4 The structure of a highly condensed, replicated chromosome.© 2003 John Wiley and Sons Publishers

• Chromosomes are paired together.

• You get one from each pair from each parent.

• The ‘map’ of the pairs is a karotype

• Diploid is the normal number of chromosomes in an autosome cell – e.g. 46 in humans.

• Haploid is the normal number in a sex cell e.g. 23 in humans

RECAP

Structure

• Sugar is deoxyribose, 5 carbons.• Four bases: two purines (adenine and guanine)

and two pyrimidines (cytosine and thymine).• Shape double helix held by weak hydrogen

bonds• The nucleotide strands run in opposite

directions – link is via the sugar so one runs 3’ to 5’ and vice versa. D

Function

• Carries genetic code for proper functioning of organisim

• Has ability to replicate itsself

• Very long and has genes positioned in set places called loci/locus.

• Genes code for a polypeptide chain One or more polypeptide chains make a

protein D

Packaging

• Prokaryote=ring

• Eukaryote needs packaging as so long.

• Chromatin when unwound, chromosome when wound and looped.

• Loop around histones which form the chromatid.