DNA, PROTEIN SYNTHESIS, & MutationsChapters 12

Pages 287-308

Chapter 12: Section 1I. History

A. Frederick Griffith- 1928

- Tried to figure out how bacteria made people sick.

- Heat killed, disease-causing bacteria passed

“something” along to harmless bacteria.

- He called this process “Transformation.”

B. Oswald Avery- 1944

- Determined DNA is the molecule of inheritance and not proteins.

– Did this by destroying other cell parts

piece by piece.

C. Alfred Hershey & Martha Chase- 1952

Hershey & Chase – determined that DNA transmits genetic information from parent to offspring, & not any other molecule

• Worked with bacteriophages – viruses that invade bacterial cells

• Used bacteriophages with radioactive markers (1952)

Structure of Bacteriophage

• Protein coat –outer covering

• DNA – inner core

Hershey & Chase

• One bacteriophage had radioactive phosphorus-32 in its DNA & was used to infect a bacterial cell

• The other had radioactive sulfur-35 in its protein coat & was used to infect a bacterial cell

• Both bacterial cells were examined• Only the bacteriophage with P-32 was inside

the bacterial cell, proving that DNA is the material that transmits genetic info

Hershey and Chase

D. Erwin Chargaff- Late 1940’s

• Found there are 4 bases in a DNA molecule– Adenine, Guanine, Cytosine and Thymine

• Found that the amount of A = T and G = C

E. Rosalind Franklin (1952)

• Franklin used X-ray diffraction to get information about the structure of DNA

• Was able to tell that DNA had an “X” like structure where the strands are twisted around each other

• All of these discoveries

led to the following….

F. Watson and Crick

- The discovery of the structure of DNA was made in 1953 by two scientists named James Watson & Francis Crick.

- Watson & Crick proposed that DNA is shaped like a “twisted ladder.”

- This twisted ladder is also called a “Double Helix.”

- They used the

findings of the other

scientists for their model.

II. DNA

A. What is DNA ?

• Stands for Deoxyribonucleic acid

• Functions:

1. Transmit genetic info. to

next generation

2. Controls all cell activities,

including protein synthesis

B. Structure of DNA

- DNA is made of nucleotides.

1. Nucleotides consist of the following:

a. Deoxyribose – a sugar

b. A Phosphate Group

c. A Nitrogen Base

1. Four possibilities

a. Adenine (A)

b. Guanine (G)

c. Cytosine (C)

d. Thymine (T)

- These nucleotide bases join together to form a long single strand.

- Each long single strand of nucleotides connects to “another” single

strand of nucleotides.

- The two strands are held together by hydrogen bonds.

C. Base Pairing Rules

1. Adenine always pairs with Thymine.

2. Cytosine always pairs with Guanine.

- The different order of these bases is what makes organisms different.

Example:

Strand 1: A – G – T – T – C – T – A – G Strand 2: T – C – A – A – G – A – T – C

Sample Exercise:

Strand 1: C – G – A – T – G – T – A – C

Strand 2: – – – – – – –

Sample Exercise:

Strand 1: T – C– A – A – C – G – C – C

Strand 2: – – – – – – –

- The more closely related two organisms are the more alike the order of their nucleotides in their DNA will be.

Example:

Humans Chimps Gorillas Orangutans

Purines vs. Pyrimadines

• Adenine and Guanine are PURINES• Thymine and Cytosine are PYRIMIDINES

• Remember:

Purines and Pyrimidines

• Purines can only pair with pyrimidines due to their size and shape.

III. DNA Terminology

A. Chromosomes

- Tightly compacted strands of DNA found when a cell is dividing! (Needs to be compact to fit in a small space!)

- Hold all genetic information.

- Chromosomes are passed on to an offspring by its parents.

Examples: Fruit Fly = 8

Humans = 46 Shrimp = 254 Chimps = 48 Chicken = 78

Gorilla = 48 Wolf = 78

B. Chromatin

- When a cell is not dividing, DNA is in the form of chromatin.

- Loosely packed DNA that is wrapped around proteins (called histones)

C. Genes

- A section of DNA on a chromosome that codes for a specific protein.

IV. DNA Replication SECTION 2

A. DNA is copied before a cell divides so that each new cell has it’s own

genetic copy.

B. There are 4 main steps:

STEP 1:

- DNA is unzipped by the enzyme HELICASE and now two single strands begin to unwind.

- Hydrogen bonds are broken.

- This is called a replication fork

STEP 2:

- Each unwound strand of DNA acts as a template to produce two new strands

of DNA.

STEP 3:

- An enzyme named DNA Polymerase will read each unwound strand and join

new complimentary nucleotides to each.

STEP 4:

- This occurs until the whole strand is

replicated.

- Now there are two

identical DNA molecules.

- Each strand contains

one original and

one complimentary

strand.Click image for animation!