nucleic acids

what is the molecule of inheritance?

• Must be able to replicate accurately

• Must contain in a stable form, the information about an organisms structure and function

• Must be able to change in order to generate variation

inheritance

Gregor Mendle - Pea guy - hereditary

Proposed the existence of particulate unit factors for each hereditary trait but the molecule was still unknown

chromosomal theory of inheritance

Walther Flemming discovered thread-like structures called chromatin within the nucleus

Theodor boveri studied chromosomes replicating and dividing (in sea urchins)

Each cell has to have the correct number of chromosomes

Chromosomes hold the key to the inheritance of characteristics

chromatin

Chromatin=DNA+protein

DNA wraps around assosiated proteins called histones

Holecule wraps around itself to form dense chromosomes

what is molecule of inheritance?

1928 - Frederick Griffiths - showed presence of a 'transforming factor' that could pass on new characteristics

Took two strains of bacteria - R & S

R-benign

S-lethal

If he heat treated S strain - mouse lived - heating killed disease + characteristic

Mixed heated S and R - lethal phenotype is restored

Characteristics can be transferred from one cell to another

the lethal phenotype could be destroyed by heat

but the lethal phenotype can be transferred to a living presiously non-lethal bacterium

1944 - Avery, McCarty and McCleod - showed that the transforming factor was DNA

importance of nucleic acids

• Key molecules in central dogma of biology

• DNA is molecule of inheritance

• Individual variation is encoded within our DNA - expressed through the conversion of RNA to protein

• Individual genetic variation is key to forensic DNA profiling

• Individual genetic variation is key to health - many diseases have underlying genetic cause

structure of DNA - Watson and Crick and Rosalin Franklin

1. Watson & Crick - proposed DNA structure and double helix

• Pentose sugar

• Nitrogenous bases

• Phosphates

 Maurice Wilkins and Rosalin Franklin studied DNA by x-Ray diffraction and concluded that it was helical

Chargaff’s Rule

1950's Chargaff carried out base composition studies on DNA

• 4 nitrogenous bases

• 50% of bases were purines and 50% pyrimidines

• Amount of adenine was equal to thymine and cytosine equal to guanine

DNA structure

Pentose sugar = deoxyribose

Four types of nitrogenous bases, join to the primary carbon in deoxyribose

• Purines - adenine and guanine - 9 carbon, double ringed structures

• Pyrimidines - thymine and cytosine - 6 carbon, single ringed structures

bases attach via covalent (glycosidic) bond to sugar molecule - at C1 position and nitrogen of base

nucleoside (base and sugar)

Deoxyribose and nitrogenous base - covalently bonded

• Deoxyadenosine

• Deoxyguanosine

• Deoxythymidine

• Deoxycytidine

nucleotide (base, sugar and phosphate)

Phosphates joined to hydroxyl on C5 of sugar group

Nucleotides have either one, two or three phosphate attached

Deoxynucleotide triphosphates (dNTPs) are the building blocks of the DNA molecule

joining nucleotides together

3' hydroxyl and 5' phosphate react to form ester bond releasing 2 phosphate groups

Leads to formation of phosphodiester bond

At end of each DNA chain the 5' sugar has free phosphate group whereas 3' sugar has free hydroxyl group

DNA strands have direction

Sugar phosphate backbone

DNA as a double stranded molecule

• Antiparallel strand held together by base pair bonding

• Bases lie perpendicular to axis, bound together by hydrogen bonds

• Two hydrogen bonds = adenine - thymine

• Three hydrogen bonds = guanine - cytosine

Watson and Crick Model

• Two polynucleotide chains wound around each other in a right handed double helix

• Sugar-phosphate backbone is on outside of helix

• Bases point to central axis

• Chains run antiparallel

 

Amount C=G

Amount T=A

Amount purine (A+G) = amount pyrimidine (T+C)

how does DNA replicate accurately?

 

If one chain is given we can fill in the opposite because of base pairing

 

DNA Replication - proposed models

Conservative model - original DNA strands separate, copied rejoin respectively

Semi-conservative model - one of each in new

Dispersive model - broken into sections and stitched back together

Meselson and Stahls experiment - semi conservative replication

using heavy and light nitrogen

If the conservative hypothesis was correct no hybrid forms would be detected

• If the dispersive hypothesis was correct DNA of intermediate density would be detected

• DNA Replication is semi-conservative

DNA Replication

Occurs in nucleus

Carried out by DNA Polymerase iii

One strand used as template to synthesis a new strand based on Chargaff's base pairing rules

New strand synthesised in 5' to 3' direction

enzymes in DNA replications

Helicase - uncoils separates

DNA polymerase - inserts and forms bonds of nucleotides

Primase - adds RNA primers

Ligase - bonds fragments

DNA replication

DNA polymerase iii synthesised new DNA strand from multiple start points - forms bubbles in DNA

Replication rate is 500-5000 bp/min in eukaryotic

Error rate = less than 1 in a billion bases

function of DNA

Some sections of DNA contain nucleotide sequences that determines order of amino acids in a protein (genes)

DNA sequence converted by transcription to an intermediate molecule - RNA

structure of RNA

Single stranded

Ribose instead of deoxyribose (2' hydroxyl group)

Uracil instead of thymine

RNA

Sugar phosphate backbone joined 5'-3'

Bases attached at the primary carbon

Synthesised from DNA in transcription

transcription

Occurs in nucleus

H bonds break and DNA unwinds

RNA polymerase read 5'-3' joining ribonucleotides according to their complementary bases

Single stranded RNA molecule released

There are 3 types of RNA

ribosomal RNA (rRNA)

Most abundant type of RNA

Combines with proteins to form ribosomes found in cytoplasm and on the RER

Ribosomes hold mRNA molecules in place for translation

Peptidyl transferase activity of rRNA catalyses formation of peptide bonds

transfer RNA (tRNA)

Smallest form of RNA (73-95 nucleotides)

Forms clover leaf structure

Carries specific amino acids (bound to 3' end) to the ribosome

tRNA 'reads' genetic code

messenger RNA (mRNA)

contains instructions for making proteins, following transcription mRNA molecules are processed, ready for translation

DNA transcribed into RNA

DNA is double stranded:

• One strand carries code for RNA - known as coding or sense strand

• Opposite (non-coding) strand acts as template for RNA polymerase

• Resulting RNA has same sequence as coding strand (but w U instead of T)

how is information encoded?

Crick and Brenner (1961) induced mutations into Bacteriophage DNA = Three nucleotides code for one amino-acids

cracking the code

In 1960's Marshall Nirenberg and others cracked the code

• Synthetic RNA using all 64 possible triplet codes identified which triplets coded for which amino acids

• The code is degenerate (one amino acid may be coded by more than one triplet of bases

mRNA is translated into protein

Requires coordination of all three types of RNA:

• mRNA = the sequence to be translated

• tRNA = carrier molecules that bring amino acids together

• rRNA = structural and enzymatic component of ribosomes, rRNA has Peptidyl transferase activity (the only non-protein enzyme) which catalyses the formation of peptide bonds between amino acids

The order of bases in mRNA molecule (the genetic code) determines the order of amino acids in a protein (its primary structure)

translating the genetic code

Several signals involved

• Initiation codon - usually AUG

• Codons - triplet order of bases in mRNA determines order of amino acids in protein

• Stop codons - UAA, UGA, UAG

The tRNA anticodon binds to its complimentary triplet (codon) in the mRNA inserting the correct amino acid into the protein

translation

mRNA held on the ribosome allowing tRNAs to bring amino acids together and create peptide bonds between amino acids

the genetic code

64 possible triplet combinations

Redundancy: >1 triplet codes for the same amino acid

3rd base 'wobble': the first two bases of the codon usually determine the amino acid

the double helix

Replication - strict base pairing gives a simple mechanism for making an exact copy

Storage of information - the order of bases form the triplet genetic code (codon) carrying the instructions to make a specific protein

Stability - strong phosphodiester bonds keep the sequence intact, weak h bonds allow message to be read

Variation - in DNA sequence results in different protein sequences (alleles)

 

nucleic acids in forensic science

All cells will contains RNA (but its presence may be transient) - limited application in forensic biology

All cells (except red blood cells) contain a nucleus therefore could provide DNA for analysis such as:

• Blood

• Semen

• Saliva

• Bone

• Teeth

• Hair

• Tissue

DNA in forensic science

DNA sequence (the order of nucleotides) vary from person to person

There are enough differences to allow us to identify an individual

• Link a suspect to bio evidence left at a scene

• Identify missing persons through matching to personal items

 Variants are inherited from parent to offspring

• Paternity testing

• Kinship testing - missing persons through relatives