DNA

The Molecule of Life

What is DNA?

DeoxyriboNucleic Acid Chargaff’s Law

A=T, G=C R. Franklin and M. Wilkins

Crystal X-ray J Watson and F Crick

Model of DNA Double stranded structure Bases inside

What is DNA Made of?

Deoxyribose sugar Phosphate Base

Purine A, G

Pyrimidine T, C

What are the Structures of the Bases?

Purines Adenine

Guanine

What are the structures of the bases?

Pyrimidines Thymine

Cytosine

Assembly of the parts

Purines and pyrimidines form chemical bonds with deoxy-ribose (5-carbon) sugar. The carbon atoms on the sugar are designated 1', 2', 3', 4' and 5'.

It is the 1' carbon of the sugar that becomes bonded to the nitrogen atom at position N1 of a pyrimidine or N9 of a purine. RNA contains ribose.

The resulting molecules are called nucleosides and can serve as elementary precursors for DNA (and RNA synthesis)

Nucleosides (examples)

Nucleosides become Nucleotides

Nucleosides form bonds with phosphate groups.

Phosphate groups bind to the 5’ C of the deoxyribose sugar.

Nucleosides + phosphate group = NUCLEOTIDE

A Nucleotide

A, G, C or T

What makes DNA Different from RNA?

Forms sugar Phosphate Backbone

A Single Strand of Nucleotides

The nucleotides connect by a series of 5' to 3' phosphate-deoxyribose bonds.

Note the sequence of the bases in the next diagram.

Polynucleotide sequences are referenced in the 5' to 3' direction

Polynucleotide

Polynucleotide pairs are Complementary:

One strand of DNA is arranged 5’ to 3’The partner strand is arranged exactly 3’

to 5’.Chargaff’s law states A = T and C=GThe strands are held together by H-

bonds between the bases

Base pairing: how it works

Hydrogen Bonding between bases:A-T Bonding

2 hydrogen bondsG-C Bonding

3 hydrogen bonds

H-Bond Orientation

H-bonds between O```H and N```H

Orientation in space

PAIRING of A with T,

PAIRING of G with C

Double Stranded DNA: Complementary strands (cont.)

The bases pair COMPLEMENTARY to one another.

Use

This complementarity allows for DNA replication and transcription

DNA:

Two complementary strands of polynucleotides

Like a zipper but held together by H-bonds (which really are not bonds, but forces)

DNA: The Double Helix

Like a ladder twisted about its axis

Each cell in our body contains 2 m worth of DNA.

The CODE

The specific base pairing and the sequence of the bases are significant.

We call the specific arrangement of bases the CODE The sequences of code form the GENE for a

specific trait. Genes are special sequences of hundreds to thousands of nucleotide base pairs that form templates for protein making

It codes for specific RNA bases for the making of specific proteins for the trait.

GENE

Exon: regions that form the code for the trait

Intron: regions that are part of the gene but are excised

Genes

Total number of genes is unknown, it estimated to be 30 000 to 120 000

Genes comprise only 3% of the chromosome—the rest is called junk DNA—its code is meaningless “junk”

What is important about base pairs?

Can predict sequence of one strand based on the sequence of the other because it is complementary

Replication and Transcription: a single strand of DNA acts as a TEMPLATE for a new strand, or for making RNA.

Repair of damaged DNA—the template DNA allows for repairs.

DNA: From Chromatin to Chromosome

DNA supercoils around tiny proteins called HISTONES.

The resulting strand with histones supercoils on itself.

Size

CHROMOSOMES

The supercoiled DNA further coils until it further supercoils as chromatin.

This is how 2 m of DNA can be packed into the nucleus of a single body cell.

At interphase of MITOSIS or MEIOSIS I, the DNA replicates itself. The chromatin become visible as double stranded DNA (DNA that has replicated).

Chromosomal Wrapping

Replication:

Why? When cells replicate, each new cell needs it’s own copy of

DNA. Where?

Nucleus in Eukaryotes. Cytosol in Prokaryotes

When? S phase of cell cycle

What? Many proteins: major is DNA Polymerase

How?

Replication

How?5’3’ directionalityStarts with RNA primerLeading StrandLagging Strand

Okasaki FragmentsSequence determined by basepairing

Nova-Cracking the Code of LifeThe Structure of DNA

Transcription

DNA RNA What is the difference between DNA and

RNA? Ribose Sugar Uracil not thymine

Transcription

Where? Nucleus in Eukaryotes Cytosol in Prokaryotes

What? RNA Polymerase plus some minor proteins

When? When RNA is needed

Why? RNA’s serve many important functions in cells

How?

Transcription

How?5’3’ directionalityUsually only one strandUses Base-pairingSame idea as with DNA replication

RNA Synthesis Animation

Translation

What? RNA Protein

Where? Cytosol

When? When proteins are need, after RNA is made

Why? Proteins are vital for cells

How?

Translation

How?Ribosomal Subunits

Small subunitLarge subunit

CodonTriplet code used

tRNA, rRNA, mRNATranslation Animation

The Genetic Code

Why is this important?

Genetic EngineeringGene Splicing

MutationsCloning

In Summary

1. A nucleotide is made of three parts: a) A phosphate group

b) A five carbon sugar (deoxyribose)

c) And a nitrogen containing