DNA and RNA

Chapter 12

Griffith Studied disease in

humans in 1928 Wanted to know

how certain types of bacteria cause pneumonia

Found two strains of pneumonia bacteria

Smooth coated bacteria caused the disease to appear in humans

Rough coated bacteria of the same type did not

Rough Strain

Smooth Strain

Heat-killed smooth

Live rough and heat killed smooth

Transformation Somehow the heat-killed bacteria

had passed their disease-causing ability to the harmless strain.

One strain of bacteria was “transformed” into a disease-causing strain.

Griffith called the process transformation

Bacteriophage Phage-eat Bacterio- Composed of a

DNA or RNA core and protein coat

Injects viral genes into bacteria and “reprogram” the bacteria to make more viruses

Hershey-Chase If they could determine which part of

the virus, the protein coat or the DNA core, entered the infected cell they would know whether genes were made of protein or DNA

They grew viruses in cultures of radioactive isotopes found in protein coats and in DNA

Sulfur 35

DNA Long molecule made up of units

called nucleotides Each nucleotide is made up of three

basic components Sugar (deoxyribose) Phosphate group Nitrogenous (nitrogen-containing) base

DNA, It’s not just for cells anymore

Nucleotides

Nucleic AcidDeoxyribose

H - Bonds Return!Chargraff’s Rules C=G in almost any

sample of DNA A=T in almost any

sample of DNA From bacteria to

humans, it’s always true

Hydrogen Bonds Return! Hydrogen bonds can form between

certain nitrogenous bases and provide just enough force to hold the two strands together

H-bonds form only between given pairs A-T and C-G

This is known as base pairing Adenine-Thiamine, Cytosine-Guanine

Chromosomes

Eukaryote Prokaryote

Prokaryotic DNA The prokaryotic DNA molecule is

located in the cytoplasm. It is usually a single circular

molecule It contains nearly all of the cell’s

genetic information Usually referred to as the cell’s

chromosome.

Miles and Miles of Info! E. coli contains

4,639,221 base pairs and is around 1.6mm

In larger terms, that’s like stuffing 300m of rope into your book bag

Bacterial DNA is usually about 1000 times as long as the bacteria itself

Chromosome Structure Many eukaryotes have as much as

1000 times the amount of DNA as prokaryotes.

The nucleus of a human cell contains more than 1 meter of DNA. (3.28 feet or 39.37 inches in each nucleus!)

Eukaryotic chromosomes contain both DNA and protein, packed together to form chromatin.

Chromatin is DNA that is tightly coiled around proteins called histones.

Histone

To make a chromosome The DNA and histone molecules form

a bead-like structure called a nucleosome.

During most of the cell cycle the fibers are dispersed and not visible.

During mitosis, the fibers are drawn together to form a tightly wound chromosome.

Supercoils

Coils

DNA

Histone

Supercoils

Nucleosome

Chromosome

Nucleosomes seem to be able to forld enormous lengths of DNA into the tiny space available in the cell.

The histone proteins have hardly changed during evolution, probably because mistakes in DNA folding could be devastating to a cell

Coils

DNA Replication: Why Multiply?

Mitosis and Meiosis both require replication of DNA molecules

Growth Healing from injury Replacing worn molecules

Creating Sex Cells for sexual reproduction

Replication

1. Before a cell divides, it duplicates its DNA in a copying process called replication.

2. The two DNA strands are “uncoiled” and “unzipped” from each other by enzymes regulating replication

1. “Unzipped” means the hydrogen bonds holding the opposing bases together are broken

3. DNA polymerase builds a new complementary strand onto each of the old unzipped strands

DNA Replication In prokaryotes,

DNA replication begins at a single point in the chromosome

Replication proceeds in two directions around the circle

In eukaryotes, DNA replication occurs at hundreds of places

Replication proceeds in both directions

Sites where separation and replication occur are called replication forks

Replication Fork

Original DNA strand

New DNA strands

DNA Polymerase

“Unzipping” -breaking Hydrogen bonds

Polymerase Joins individual

nucleotides to produce a DNA molecule (which is a polymer)

“Proofreads” each new DNA strand to maximize odds of correct pairing

DNA