11. Nucleic Acids / Genes. Chapter 18. Nucleic Acids – the Master Molecules. Nucleotide:Phosphate + Sugar + Heterocycle;nucleoside: No phosphate. DNA - DeoxyriboNucleic Acid RNA - RiboNucleic Acid. A denosine. T hymidine. G uanidine. C ytidine. A Sequence of DNA.
11. Nucleic Acids / Genes
Nucleic Acids – the Master Molecules
Nucleotide:Phosphate + Sugar + Heterocycle;nucleoside: No phosphate
DNA - DeoxyriboNucleic AcidRNA - RiboNucleic Acid
A Sequence of DNA
Base-Pairing - the ‘Glue’ of the Double Helix
The DNA of each human cell contains ~6 billion of these base-pairs
DNA - Storehouse of the Genetic Code
The Double Helix
Chromosomes - chains of DNA contained in the nucleus of every cell. Arranged in 23 pairs (of each pair, 1 set comes from sperm and 1 from egg).
Total length of these molecular threads in each cell = ~2 meters !
When cells divide one strand from each ‘double thread/helix ’ goes to each new cell thus carrying the identical sequence/information.
The sequence of bases A, C, T & G contains the information to direct the synthesis of all the proteins in the body and is called the Genetic Code.
The sequence of bases, somewhere on a chromosome, that is responsible for each protein is called a gene.
If there are only 4 ‘bases/letters’ which must uniquely code for 20 different amino acids then the relationship cannot be 1:1 or 2:1 but must be 3:1, ie. a triplet code.
Building a new DNA Chain (Replication)
The GENETIC CODE - a Proposal and a Nobel Prize
First published by James Watson* and Francis Crick in 1953. Nobel prize awarded in 1962 to Watson,Crick and M. Wilkens (based on X-ray results by Rosemary Carter).
* head of the World Genome Project, ~1990 -2001.
Bedtime reading: The Double Helix (J. Watson)
The Genetic Code
AUG also = start
The Human Genome contains more than 100,000 genes each of which can be 1000 - 100,000 units (base-pairs) long ......... but ..........
this is only ~3 - 5% of the total number of units available!
Why? - maybe safety How to find? - start/stop signals
transfer RNA: withanticodonand related amino acid
a.a. binding site
The human body can repair DNA/RNA by cutting, splicing, inserting, but mutations can occur !
Mutation - any chemical or physical change that alters the nucleic acid sequence in the DNA.
May be by chemical means, radiation, etc. May be by substitution, insertion, deletion.
Every time a cell divides ~6 billion NA are matched and ~ 2000 errors occur (most are repaired).
'Aging' is thought to be due to an increased breakdown of RNA.
Most mutations are detrimental. If at a crucial position the defective protein will lack biological activity, the cell/organism dies and the DNA will not be reproduced.
Non-lethal mutations often lead to metabolic abnormalities or hereditary/genetic diseases,
eg. sickle-cell anemia, hemophilia or PKU (phenylketonuria) - cannot convert Phe to Tyr (precursor of neurotransmitters); can cause severe mental retardation.(~1 in 12,000). But can be cured/controlled if detected within 3 weeks of birth.