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=G The strands are held together by H- bonds between the bases

Base pairing: how it works Hydrogen Bonding between bases: A-T Bonding 2 hydrogen bonds G-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 to 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’ directionality Starts with RNA primer Leading Strand Lagging Strand Okasaki Fragments Sequence determined by basepairing

Nova-Cracking the Code of Life The 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’ directionality Usually only one strand Uses Base-pairing Same 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 Ribosomal Subunits Small subunit Large subunit Codon Triplet code used tRNA, rRNA, mRNA Translation Animation

The Genetic Code

Why is this important? Genetic Engineering Gene Splicing Mutations Cloning

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