Topic 7.1: DNA structure

7.1.1 Describe the structure of DNA, including the anti-parallel strands, 3'-5' linkages and hydrogen bonds between purines and pyrimidines

DNA

DNA has a double stranded helix which has uniform diameter along its entire length. The two polynucleotide strands are "anti-parallel", they run in opposite direction.

The hydrogen-bonding is specific and known as complementary base pairing. Adenine and Thymine are double hydrogen bonded while Cytosine and Guanine are triple hydrogen bonded.

Purines are double ringed nucleotides; Adenine and Guanine
Pyrimidine are single ringed nucleotides; Cytosine and Thymine


7.1.2 Outline the structure of nucleosomes

The double helix has a lot of major and minor grooves on its outer diameter, this exposes chemical groups to form hydrogen bond with it.

The DNA is bonded to 8 histone molecules (2 x 2 x 2). The combination of DNA and histones is secured by the "H1 linker" protein.


7.1.3 State that nucleosomes help to supercoil chromosomes and help to regulate transcription

Nucleosomes

• Protect DNA from damage
• They allow long lengths of DNA to be packaged (super-coiled) for mobility during meiosis/mitosis
• However when super-coiled, the DNA is not accessible for transcription
• It condenses the DNA to by a factor of 15000x

7.1.4 Distinguish between unique or single-copy genes and highly repetitive sequences in nuclear DNA

7.1.5 State that eukaryotic genes can contain exons and introns

Exons are the part of the gene which codes for a protein (EXpressing sequence)

Introns are the non-coding sequence of DNA within a gene (INtervening sequence) that is cut out by enzymes when RNA is made into mature mRNA

Eukaryotic organism have DNA which differs from prokaryotic organisms.