Higher Biology | Key Area 1.1: The Structure of DNA

Nucleotide

The basic unit of DNA, comprised of a deoxyribose sugar (3’ end), organic nitrogenous base, and phosphate group (5’ end).

The four nitrogenous bases

- Adenine = Thymine (two weak hydrogen bonds)

- Cytosine ≡ Guanine (three weak hydrogen bonds)

DNA strands

- The two DNA strands run antiparallel, ie. one runs 5’→3’, the other 3’→5’, creating a twisting double helix structure.

- DNA nucleotides in a strand of DNA are joined by strong chemical bonds between the phosphate and deoxyribose of separate nucleotides, creating the sugar-phosphate backbone.

Genetic code

- The base sequence of DNA forms the genetic code. Genetic information is stored in these base sequences in all cells.

- Genotypes are determined by the base sequence of DNA.

What can be said about the organisation of DNA?

DNA is organised differently in different types of organisms.

Eukaryote

An organism whose cells have chromosomes enclosed within a nucleus, and has linear chromosomes, eg. animals, plants, fungi.

Organisation of DNA in eukaryotes

- DNA found in linear chromosomes within the nucleus is tightly coiled and packaged around equal masses of associated histone proteins, forming a repeating array of DNA protein particles (nucleosomes).

- Nucleosomes are cores of histone proteins, around which the double helix is wrapped.

- Mitochondria and chloroplasts have circular chromosomes regardless.

Prokaryote

A unicellular organism which lacks a membrane-bound nucleus, have a singular circular chromosome, and smaller circular plasmids, eg. bacteria.

Organisation of DNA in prokaryotes

- DNA is free in the cytoplasm due to the absence of a membrane-bound nucleus.

- Have a singular circular chromosome (usually in the nucleoid region) which holds all genes needed for that organism.

- Have plasmids holding extra circles of DNA, which contain additional genes to increase an organism’s chance of survival (eg. antibiotic resistance).

Yeast is a special eukaryotic cell. Why?

Yeast is a eukaryotic unicellular cell, which has linear chromosomes, but also contains plasmids, like prokaryotes.