Biology DNA Structure

Deoxyribonuucleic acid is a nucleic acid. It is a polymer of sub--units called nucleotides. It consists of 2 polynucleotide strands held together by hydrogen bonds to form an alpha double helix.

A gene is a section of DNA that contains coded information as a specific sequence of bases.

Genes code for polypeptides that determine the nature and development of an organism.

Each nucleotide consists of 3 molecules joined by a condensation reaction:

• A pentose sugar
• a phosphoric acid molecule
• an organice base

A+T - Complimentary + Pyrimidine

C+G - Complimentary + Purine (double ringed bases)

• Sugar-phosphate backbone - gives strength
• Coiling of DNA - gives compact shape
• Double helix - protects sequence of bases + keeps molecule more stable
• Large molecule - Allows a large amount of information to be stored
• Many hydrogen bonds - give stability + prevents code being interrupted + allows chain to easily unzip for replication
• Sequence of bases - codes for synthesis of specific proteins
• Complimentary Base Pairing - enables information to be replicated accurately

An allele is a gene existing in a different form

A codon - these code for specific amino acids

• differ in their base sequence + code for different sequence of amino acids hence different polypeptides are produced
• This change in polypeptides makes the protein non-functional as there is a change in the tertiary structure of the protein
• This is because a change in the base sequence affects the hydrogen, ionics and (if present) disulfide bonds
• If the protein coded for is an enzyme, the shape of the active site may be altered and therefore the enzyme is non-functional as the substrate cannot attach and an enzyme-substrate complex cannot be formed

Haploid = (n) 23

Diploid = (2n) 46

Interphase - represents the non-dividing cell when cell growth occurs

Nuclear division - the nucleus divides into 2 (mitosis) or 4 (meiosis)

Cell division - The cell divides into 2 (mitosis) or 4 (meiosis)

During late interphase the cell prepares for division in the following ways:

• There is an increase in protein synthesis inside the cell
• The DNA content replicates via DNA replication
• Cell organelles are replicated

Chromosomes form and become visable. Each chromosome consists of 2 identical chromatids.

Each chromosome shortens and thickens

Centrioles move to opposite poles of the cell and nuclioli and nuclear membrane break down

Centrioles form a spindle across the cell - the spindle consists of protein microtubules

Each chromosome moves to the equator of the spindle and attaches to it via its centromere