DNA Structure DNA consists of two molecules that are arranged into a ladder-like structure called a Double Helix. A molecule of DNA is made up of millions of tiny subunits called Nucleotides. Each nucleotide consists of: 1. Phosphate group 2. Pentose sugar 3. Nitrogenous base

Nucleotides Phosphate Pentose Sugar Nitrogenous Base

Nucleotides The phosphate and sugar form the backbone of the DNA molecule, whereas the bases form the “rungs”. There are four types of nitrogenous bases.

Nucleotides A Adenine T Thymine G Guanine C Cytosine

Nucleotides Each base will only bond with one other specific base. Adenine (A) Thymine (T) Cytosine (C) Guanine (G) Form a base pair.

DNA Structure Because of this complementary base pairing, the order of the bases in one strand determines the order of the bases in the other strand.

G G A T T A A C T G C A T C

DNA Structure To crack the genetic code found in DNA we need to look at the sequence of bases. The bases are arranged in triplets called codons. A G G - C T C - A A G - T C C - T A G T C C - G A G - T T C - A G G - A T C

A gene is a section of DNA that codes for a protein. Each unique gene has a unique sequence of bases. This unique sequence of bases will code for the production of a unique protein. It is these proteins and combination of proteins that give us a unique phenotype.

Protein DNA Gene Trait

Cellular DNA Content and the Structural or Organizational Complexity of the Organism

The Molecular Structure of the Eukaryotic Chromosome A eukaryotic cell contains a large amount of DNA in its nucleus Eukaryotic chromosomes consist of one linear, unbroken, double stranded DNA molecule along its length The nuclear chromosomes of eukaryotes are complexes of DNA, histone proteins, and nonhistone chromosomal proteins, and RNA (called chromatin) By weight, eukaryotic chromosomes contain approximately twice as much protein as DNA

The Molecular Structure of the Eukaryotic Chromosome The large amount of DNA present in eukaryotic chromosomes is compacted by association with histones, forming structures called nucleosomes Nucleosomes fold further into chromatin fibers Each chromosome contains a large number of looped domains of 30-nm chromatin fibers attached to a protein scaffold

CHROMOSOMAL PROTEINS HISTONES Small basic proteins Constant amount in cells 25% lysine & arginine (Net + charge) 5 main types: H1, H2A, H2B, H3 & H4

CHROMOSOMAL PROTEINS Equal amount of histones & DNA H2A, H2B, H3 & H4 are highly conserved among distinct species Histone proteins are among the most conserved proteins H1 varies in cells (in RBC it is replaced by H5)

CHROMOSOMAL PROTEINS NON-HISTONE PROTEINS All DNA chromosomal proteins minus histones Structural proteins or enzymes i.e. DNA replication enzymes, regulatory proteins, transcription factors… Differ in number and type in different cell types

CHROMOSOMAL PROTEINS Acidic proteins (negatively charged) Equal amount of non-histones & DNA Example of HMGs (High-Motility Group proteins) Bind to minor groove Have a role in DNA bending Have a role in formation of higher order chromatin structure

NUCLEOSOME Octamer of histones 2 (H2A, H2B, H3, H4) + linker histone H bp of DNA DNA compacts by winding 1 and ¾ turn of the outside of the histone octamer Under electron microscopy, 11 nm chromatin fiber (beads on a string)

Nucleosome Structure

Nucleosomes connected together by linker DNA and H1 histone to produce the “beads-on-a- string” extended form of chromatin

Packaging of nucleosomes into the 30-nm chromatin fiber

Model for the organization of 30-nm chromatin fiber into looped domains that are anchored to a nonhistone protein chromosome scaffold

The many different orders of chromatin packing that give rise to the highly condensed metaphase chromosome (700 X compaction)

10 nm Fiber A string of nucleosomes is seen under EM as a 10 nm fiber

30 nm Fiber 30 nm fiber is coil of nucleosomes with 6/turn

Orders of chromatin structure from naked DNA to chromatin to fully condensed chromosomes... Fig. 9

The Molecular Structure of the Eukaryotic Chromosome The functional state of the chromosome is related to the extent of coiling The more condensed areas of the chromosome (heterochromatin) are genetically inactive The less compacted regions (euchromatin) contain genes that are expressed

Euchromatin (E) vs Heterochromatin (H) E H DNase Being more condensed (tightly packed), heterochromatin is resistant to DNase digestion. Fig. 11

Hypothesis: Transcriptionally active DNA (i.e., an active gene) is in euchromatin. Can test with DNase I, which should preferentially digest active genes in a cell... E H nascent transcripts

Euchromatin and Heterochromatin