Chapter 4: Structure and Function of DNA

Overview:Nucleic acid chemical compositionChromosome three dimensional structure

Goals:Understand how genes are physically organized

within cellsUnderstand the links between DNA structure and

regulation of DNA replication and transcription

DNA structureNitrogenous bases:

purines: adenine, guanine

pyrimidines: thymine, cytosine, uracil

DNA structureNucleosides are nitrogenous base

+ a sugar (deoxyribose or ribose)CH3

1

23

4

5

OHOH

Examples: adenosine & deoxyadenosine,

guanosine, thymidine, cytidine

DNA structureNucleotides are nucleosides esterified to one or more phosphates (e.g. adenosine triphosphate-ATP

or deoxyadenosine triphosphate -dATP)

DNA structureDNA is a polymerized chain of nucleotides

Pyrophosphate(PPi)

+

DNA structureStrands have a 5’ end (phosphate) and a 3’ end (sugar)

5’ end(5-prime)

3’ end(3-prime)

DNA structure

2 DNA strands anneal (hybridize): specific, complementary base pairs form hydrogen bonds to make double stranded DNA

A

T

C

G

T

A

G

C

DNA structureDNA strands are anti-parallel

DNA and genomesLong stretches of DNA make chromosomes

The collection of chromosomes within the cell is the genome.

Prokaryotes and organelles have genomes that are:

• single, circular chromosomes• no histones• functionally related genes are regulated

together as operons

Eukaryote genomesEukaryote (nuclear) genomes are:• variable number of chromosomes• histones present• single genes• enclosed in a nucleus

How do you pack 40km of DNA into a 6um2

nucleus?

Chromatin is double stranded DNA, with the associated proteins

Chromatin structure allows long strands of DNA to be condensed into ordered, accessible arrangements

Chromosome structure

Chromosome StructureChromosomes are

coated with histone and non-histone proteins

Histone octamers organize chromosomes into nucleosomes

Chromosome Structure

Non-histone proteins bind other places on chromosome

Chromosome Structure

Histones also regulate nuclear activity: changes in structure can alter gene expression:

•acetylation•phosphorylation •methylation

Chromosome numberDNA of a genome

is divided into chromosomes (variable size and number)

Chromosome structureGenes include:• Exons-coding regions• Introns- non-coding regions interspersed

between exons• Regulatory sequences

In addition to genes, chromosomes have lots of “non-coding” sequence

Chromosome structureStructural features:• Replication origin• Centromere• Telomeres

Repetitious DNA• LINES and SINES• DNA only transposons• Segmental duplications and simple sequence

repeats

Chromosome structure