DNA Structure and Organization5.1

What is DNA?

• DNA = deoxyribonucleic acid

• The discovery of DNA and its role in genetics unfolded over many years...

1)Frederick Griffith( 1879-1941)

• Frederick Griffith looked at DNA in 1928• His research was on Streptococcus pneumoniae• He looked at 2 strains of the bacteria:-> S strain vs. R strain(pathogenic) (non pathogenic)-> S strain was non pathogenic when heated…

Transformation Principle:

Fredrick Griffith showed that something was present in the heat-killed S-strain of S. pneumoniae that could transform the non-pathogenic R-strain into the pathogenic form.

• In conclusion…

Griffith’s discovery of the Transformation Principle was proven correct as researchers later discovered that DNA in the heated strain bacteria caused the transformation

2) Hershey and Chase

• In 1952, Alfred Hershey and Martha Chase sought out to discover whether DNA or protein was responsible for the passing on of hereditary material.

• Using a T2 bacteriophage strain of a virus they discovered that viral DNA was transferred to bacterial cells & viral DNA held genetic info needed for viruses to reproduce.

Pg. 207- Hershey & Chase Investigation

3) Fredrich Miescher(1869)

• Swiss chemist, isolated the nuclei of white blood cells from pus-soiled bandages– From this, he extracted nitrogen and phosphorous– Later research would confirm this as “Nucleic

acid”

4) Phoebus Levene ( early 1900s)

• Russian American biochemist, that continued from Meisher’s research to identify chemical composition of nucleic acid

• He isolated two types of nucleic acid (now called DNA and RNA)

NUCLEOTIDESIn 1919 Levene proposed that DNA and RNA are made up of single units nucleotides

Nucleotides are composed of:Sugar, phosphate group, & nitrogenous base

• DNA and RNA are made up of a combination of four different nucleotides

• DNA has 4 nitrogenous bases: adenine (A), thymine (T), cytosine (C), guanine (G)

• RNA has the same nitrogenous bases, except, thymine (T) is replaced by uracil (U)

• They are divided into two groups: pyramidines (1 ring) and purines (2 rings)

*Note: Both DNA and RNA contain the same purine bases and the cytosine pyrimidine base. However, thymine is only present in DNA, and uracil is only present in RNA.

5) Erwin Chargaff• Austrian American biochemist; in the 1940

launched research to study chemistry of nucleic acids

• His approach- How can genetic info possibly be contained in DNA?

• He also looked at difference in DNA between species

Chargaff’s Rule: in DNA the % composition of adenine is the same as thymine, and the % composition of cytosine is the same as guanine

We know this as complimentary base pairing.

6) Linus Pauling( 1901-1994)

• American chemist; developed methods of assembling 3D models based on bond angles of atoms in molecules

• In 1951 he discovered that proteins have helix shaped structures

7) Rosalind Franklin( 1920-1958)

• British Chemist; used x-ray diffraction to analyze the structure of biological molecules

• In the 1950s she concluded that DNA has a helical structure!!!!

• She also concluded that the nitrogenous bases were on the inside w/ sugar-phosphate backbone on the outside

X-Ray Diffraction

8) Watson and Crick

• James Watson and Francis Crick constructed the first DNA model

• Used conclusion of peers to help them• Concluded that DNA has a twisted, ladder-like

structure– Double helix

• Sugar-phosphate molecules make up sides of ladder

• Nitrogenous bases make up rungs of ladder• In 1953 they published a paper on this– Currently accepted model of DNA

Modern DNA Model:The Double Helix

• We can now identify the position of every atom in a molecule of DNA

Features of DNA– Two polynucleotide strands form double

helix– Backbone of alternating sugar and

phosphate groups– Two strands of DNA are complimentary (ie paired as follows: A-T, C-G)

Features cont’d…-Hydrogen bonds link A-T (2 bonds) and C-G (3 bonds)

-Each strand is antiparallel; phosphate is on the 5’ carbon and OH is on the 3’ carbon-5’ end lies across from the 3’ end

DNA in Prokaryotes vs. Eukaryotes

Genome: total genetic material of an organism (entire DNA sequence)

Gene: basic unit of molecular heredity for a specific trait (sequence of DNA that codes for protein and RNA molecules)

*Note: In an organism, most of the DNA is non-coding, meaning that it does not have any instructions for making molecules.

DNA in Prokaryotes(ie. E. coli)• Genetic material is a circular, double stranded

DNA molecule• One single chromosome (may have more than

one copy)• No nuclear membrane, so bacterial

chromosome is packed tightly within a specific region called a a nucleoid

• Bacterial chromosomal DNA is compacted ~1000x

• Specialized proteins that bind to bacterial DNA fold it into loops

• These folds compact it 10X more

DNA Supercoiling: further compacting of DNA by twisting the structure

• In bacteria, supercoiling is controlled by the enzymes:– topoisomerase I and topoisomerase II– Antibacterial drugs block the activity of the

enzymes (e.g. Quinolones, and coumarins)

• Some prokaryotes have small circular or linear DNA molecules called plasmids– Not part of nucleoid– Copied and transmitted b/t cells or incorporated

into chromosomal DNA and reproduced during division

• Most prokaryotes are haploid organisms and their genes have little non-essential DNA

• Most of genomes have regulatory sequences-Sections of DNA sequences that determine when genes are activated

E. coli genome with regulatory sequences such as lac and trp. The lac operon codes for the metabolism of lactase and the trp codes for tryptophan production.

DNA in Eukaryotes• DNA is located in the nucleus• Total amount much greater

than prokaryotes• If you lined up all the DNA in

a human, it would be 2m in length, but 4µm in diameter

• Requires more compacting than in prokaryotes

• Several levels of organization

1) DNA winds around histones

2) Bead-like structure forms a tightly packed array to produce 30 nm fibres

3) 30nm fibres form loops that are attached to protein scaffold

4) Scaffold further condenses genetic material into chromosomes

Chromatin: Genetic material condensed to form chromosomes during eukaryotic cell division

• During interphase, the level of chromatin compaction can vary along chromosome.

• During prophase the chromatin fibres become coiled into chromosomes

Variation in Eukaryotic Genome• Varies greatly between species• Most are diploid (two copies of each gene)• Some are haploid (one copy)– E.g. ferns, algae

• Some are bred to have 3+ copies– E.g. Seedless watermelons (triploid)

• The # of genes on each chromosome varies too– E.g. Chromosome 19 in humans has 72 million

base pairs/1450 genes; chromosome 4 has 1.3 billion base pair/200 genes

• There is not correlation b/t organisms complexity and amount of DNA– E.g. Lingish have 40x more DNA than humans

Learning Expectations...

• History of DNA ( main discoveries)– Griffith, Hershey and Chase, Chargaff, Pauling,

Franklin– Watson and Crick!

• Molecular structure of DNA (the fine details)• Difference in DNA b/t prokaryotic and

eukaryotic cells

Homework

• Read and make notes 5.1• Complete pg. 207 #1-4• Complete pg. 212 #7-11