Dna structure, genes and chromosomes

• DNA stores and transmits the cell's genetic information.
• Every time a cell divides, it must pass on an exact copy of all its DNA to each daughter cell.
• DNA must be stable, able to accurately copy itself (millions of times), and able to contain coded information.
• Frederick Griffith (1879–1941) conducted an experiment where a smooth strain of Streptococcus pneumonia bacteria was found to transform into a virulent state when mixed with a dead rough strain.
• In 1944, Avery, MacLeod and McCarty published a key study using cell-free extracts of heat-killed 'S-strain' bacteria to show that DNA is the chemical substance responsible for bacterial transformation.
• Rosalind Franklin and Maurice Wilkins shared the Nobel Prize in 1962 for their discovery of DNA, almost 100 years after its discovery in pus.
• The sugar-phosphate backbone is on the outside; the antiparallel strands are held together by Hydrogen bonds (H bonds).
• DNA is made up of multiple nucleotides, linked together via a sugar-phosphate backbone.
• One nucleotide attaches to another nucleotide covalently by phosphoester bonds between the 3’-hydroxyl (-OH) group of one sugar and the 5’-phosphate (-OPO3) group of the next sugar.
• DNA forms a right-handed helix with a major groove where the backbones are far apart and a minor groove where they are close together.
• DNA strands pair up by complementary base pairing, with A always pairing with T and G always pairing with C.
• The two strands of DNA run 'anti-parallel' to each other, held by hydrogen bonds between the bases on the different strands.
• A single nucleotide is linked to a base, which is nitrogen-containing, such as Adenine.
• One end of the strand finishes with the 5’ phosphate and is called the 5’ end.
• Polynucleotides are formed via phosphodiester bonds.
• The other end of the strand finishes with a free 3’ hydroxyl and is called the 3’ end.
• Multiple origins of replication permit the start of DNA replication.
• All chromosomes must be replicated (copied) when the cell divides into two daughter cells.
• The complex between DNA and protein is called chromatin.
• Genes are the instructions for making proteins: DNA → RNA → Protein.
• The centromere attaches the duplicated chromosomes to the mitotic spindle.
• The telomere is a protective cap at both ends of a linear chromosome.
• Specific proteins called histones bind to and fold the DNA into coils and loops in an organised way.
• Each human cell contains 2 metres of DNA yet the cell nucleus is only 5-8 µm in diameter.
• Eukaryotes have multiple, linear chromosomes in the cell nucleus and each species has a characteristic number of chromosomes.
• Chromatin condensation involves a single histone 1 (H1) molecule binding to both the linker DNA and to the DNA wound around the histone core, pulling the chromatin together.
• Histones are highly conserved, small, basic proteins with a net +ve charge that bind the negatively charged DNA.
• The chromosomes usually occur as 2 complementary sets, ie diploid.
• One gene can produce multiple polypeptides (proteins).
• The complete set of human chromosomes is 46, consisting of 22 pairs of autosomes and 1 of each sex chromosome.
• Eukaryotic chromosomes contain large amounts of DNA needed to encode the information to make a multicellular organism.
• Histone H1 induces tighter DNA wrapping.
• Humans have 23 pairs of homologous chromosomes; one haploid set of chromosomes from the egg and the other haploid set from the sperm.
• Two of each of four histone proteins wind 1.65 times around the nucleosome to compact the DNA overall by about 6 times.
• Nucleosomes play a key role in chromatin packing.