Chapter 12: Molecular Genetics

• What You’ll Learn• How DNA was

discovered to be the genetic material & know its structure

• DNA replication• Protein synthesis• Gene regulation &

mutations

Section 12.1: DNA: The Genetic Material

• Section Objectives:• Summarize the

experiments leading to the discovery of DNA as the gentic material

• Analyze the structure of DNA

• Describe the basic structure of the eukaryotic chromosome

What is DNA?

• Although the environment influences how an organism develops, the genetic information that is held in the molecules of DNA ultimately determines an organism’s traits.

• DNA achieves its control by determining the structure of proteins.

• Within the structure of DNA is the information for life—the complete instructions for manufacturing all the proteins for an organism.

Bodies are made up of cells All cells run on a set of

instructions spelled out in DNA

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Bodies Cells DNA

The “Transforming Principle”

1928

• Frederick Griffith – Streptococcus pneumonia

bacteria• was working to find cure for pneumonia

– harmless live bacteria (“rough”) mixed with heat-killed pathogenic bacteria (“smooth”) causes fatal disease in mice

– a substance passed from dead bacteria to live bacteria to change their phenotype• “Transforming Principle”

The “Transforming Principle”

Transformation = change in phenotypesomething in heat-killed bacteria could still transmit disease-causing properties

live pathogenicstrain of bacteria

live non-pathogenicstrain of bacteria

mice die mice live

heat-killed pathogenic bacteria

mix heat-killed pathogenic & non-pathogenicbacteria

mice live mice die

A. B. C. D.

DNA is the “Transforming Principle”

• Avery, McCarty & MacLeod– purified both DNA & proteins separately from

Streptococcus pneumonia bacteria• which will transform non-pathogenic bacteria?

– injected protein into bacteria• no effect

– injected DNA into bacteria• transformed harmless bacteria into

virulent bacteria

1944

What’s theconclusion?

mice die

Confirmation of DNA• Hershey & Chase– classic “blender” experiment– worked with bacteriophage• viruses that infect bacteria

– grew phage viruses in 2 media, radioactively labeled with either • 35S in their proteins• 32P in their DNA

– infected bacteria with labeled phages

1952 | 1969Hershey

Why useSulfurvs.Phosphorus?

Protein coat labeledwith 35S

DNA labeled with 32P

bacteriophages infectbacterial cells

T2 bacteriophagesare labeled withradioactive isotopesS vs. P

bacterial cells are agitatedto remove viral protein coats

35S radioactivityfound in the medium

32P radioactivity foundin the bacterial cells

Which radioactive marker is found inside the cell?Which molecule carries viral genetic info?

Hershey & Chase

Blender experiment• Radioactive phage & bacteria in

blender– 35S phage

• radioactive proteins stayed in supernatant• therefore viral protein did NOT enter bacteria

– 32P phage• radioactive DNA stayed in pellet• therefore viral DNA did enter bacteria

– Confirmed DNA is “transforming factor”

Taaa-Daaa!

Hershey & Chase

Alfred HersheyMartha Chase

1952 | 1969Hershey

The structure of nucleotides

• DNA is a polymer made of repeating subunits called nucleotides.(the monomer)

• Nucleotides have three parts: a simple sugar, a phosphate group, and a nitrogenous base.

Sugar (deoxyribose)

Nitrogenous basePhosphate group

The structure of nucleotides• in DNA there are four possible nucleotides,

each containing one of these four bases.• The phosphate groups and deoxyribose

molecules form the backbone of the chain, and the nitrogenous bases stick out like the teeth of a zipper.

Nucleotide

Sugar-phosphate backbone

Phosphategroup

Sugar(deoxyribose)

DNA nucleotide

Nitrogenous base(A, G, C, or T)

Thymine (T)

Chargaff

• DNA composition: “Chargaff’s rules”– varies from species to species– all 4 bases not in equal quantity– bases present in characteristic ratio• humans:

A = 30.9% T = 29.4% G = 19.9% C = 19.8%

1947

That’s interesting!What do you notice?

RulesA = TC = G

Paired bases• DNA structure– double helix• 2 sides like a ladder

• Bases match together (– A pairs with T • A : T

– C pairs with G• C : G

Structure of DNA

• James Watson and Francis Crick worked out the three-dimensional structure of DNA, based on work by Rosalind Franklin and Maurice Wilkens

1953 | 1962

Wilkins

Rosalind Franklin (1920-1958)

DNA is a double-stranded helix

• Watson and Crick also proposed that DNA is shaped like a long zipper that is twisted into a coil like a spring.

• Because DNA is composed of two strands twisted together, its shape is called double helix.

The structure of DNA

Ribbon model

Hydrogen bond

Partial chemical structure

Computer model

Basepair

Anti-parallel strands

–DNA molecule has “direction”–complementary

strand runs in opposite direction

3

5

5

3

The importance of nucleotide sequencesThe sequence of nucleotides

forms the unique genetic information of an organism. The closer the relationship is between two organisms, the more similar their DNA nucleotide sequences will be.

• Scientists use nucleotide sequences to determine evolutionary relationships among organisms, to determine whether two people are related, and to identify bodies of crime victims.

Chromosome

Organizing & packaging DNA

nucleus

cell

DNA

nucleus

cell

4 chromosomesin this organism

DNA in chromosomes ineveryday “working” cell

DNA in chromosomes in cell getting ready to divide

DNA has been “wound up”

DNA PackingDNAdoublehelix(2-nmdiameter

Histones

“Beads ona string”

Nucleosome(10-nm diameter)

Tight helical fiber(30-nm diameter) Supercoil

(200-nm diameter)

Metaphase chromosome

700nm

Nucleosomes • “Beads on a string”– 1st level of DNA packing– histone proteins • 8 protein molecules• positively charged amino acids • bind tightly to negatively charged

DNA

8 histone molecules