CHAPTER 12: DNAThe Blueprint of Life

DNA Video

Importance of DNA video clip

DNA = Deoxyribonucleic Acid

Role of DNA Molecule that stores

genetic information in living cells

Genetic information can be copied for new cells

Information is transmitted to new offspring

12.2 Structure of DNA

Objectives Identify the chemical

components of DNA. Discuss the experiments

leading to the identification of DNA as the molecule that carries the genetic code.

Describe the steps leading to the development of the double-helix model of DNA.

Components of DNA

DNA is a polymer of nucleotides These nucleotides have three parts:

Phosphate group 5 carbon sugar: deoxyribose Nitrogen base: A, G, C, T

A = adenine G = guanine C = cytosine T = thymine

Covalent bonds join nucleotides

Check-in

What are the three roles of DNA?

DNA is a polymer of _______________. A nucleotide is made of what three

things?

What type of bond joins nucleotides together?

Storing, copying and transmitting genetic information

Phosphate group, deoxyribose sugar, nitrogenous base

Covalent bond

nucleotides

Solving the Structure of DNA

Chargaff’s Rule – true for all organisms Amount of A = Amount of T Amount of C = Amount of G

Rosalind Franklin Used X-ray diffraction Took picture of DNA in 1952 Picture gave clues about DNA structure:

Strands twisted into helix (like coiled spring) Two strands in DNA Nitrogenous bases in center

Solving the Structure of DNA

James Watson and Francis Crick Built 3-D models of DNA Shown Franklin’s Photo 51 Led to discovery of double helix shape in 1953

Check-in

Describe Chargaff’s rule?

What clues did Rosalind Franklin’s picture give about the structure of DNA?

What method did she use to take the picture?

Looking at DNA in all organisms, amount of adenine equals amount of thymine and amount of guanine equals amount of cytosine.

DNA was helix shaped and contained two strands; bases are in the center of DNA molecule.

X-ray diffraction

Check-in

Who is credited with the discovery of the structure of DNA?

What method did they use to discover its structure?

James Watson and Francis Crick

Built three dimensional models.

Double-Helix Model

DNA is shaped like a double helix Looks like: spiral staircase or twisted ladder

Sides = Phosphate groups and sugars Steps or rungs = Nitrogen bases Bases are held together by weak hydrogen

bonds

Double-Helix Model

Stands are antiparallel – run in opposite directions

Which bases are complementary to each other? A with T G with C Called base pairing Explains Chargaff’s rule

Structure of DNA

Chromatin, Chromosomes and DNA subunits video clip

Check-in

What does a double helix look like?

What forms the backbone of DNA or sides of the ladder?

What forms the rungs of the ladder?

What type of bond holds the bases together?

Twisted ladder or spiral staircase

Sugar (deoxyribose) and phosphate groups.

Nitrogenous bases

Hydrogen bonds

Check-in

What base bonds with A (adenine)?

What base bonds with C (cytosine)?

What is the complementary DNA strand for the following: AGGTCT

T (thymine)

G (guanine)

TCCAGA

12.3 DNA Replication

Objectives Summarize the events of DNA

replication. Compare DNA replication in

prokaryotes with that of eukaryotes.

DNA Replication

What is it? Process by which chromosomal DNA is copied

before mitosis and meiosis

When does it happen? Interphase – S phase

Why does it happen? So each new cell gets a complete copy of DNA

Steps of DNA Replication1. Strands of DNA

Separate: Hydrogen bonds between

bases are broken by an enzyme and the strands separate

2. Base Pairing: Free-floating nucleotides

pair up with bases on the exposed strand (matches with the complementary base – AT & CG)

Steps of DNA Replication3. Bonding Nucleotides Backbone

(sugar/phosphates) of the once-free nucleotides are bonded together by an enzyme (DNA polymerase) to form a new complementary strand

4. Result of Replication Two DNA molecules, each

with a new strand and an old strandAnimation of DNA Replication

DNA Replication

Role of enzymes One enzyme unzips the strands (breaks

hydrogen bonds) DNA polymerase creates sugar-phosphate

bonds and “proofreads” new DNA strand Telomerase adds short DNA sequences to

telomeres

Telomere

Prokaryotes vs. Eukaryotes

Prokaryotes Single circular DNA molecule in cytoplasm Replication starts at one point and continues

in both directions

Prokaryotes vs. Eukaryotes

Eukaryotes Chromosomes much bigger than prokaryotes Replication starts at many places and

continues in both directions

Check-in

Describe the three steps of DNA replication?

What is the result of DNA replication?

What are the two functions of DNA polymerase?

1. DNA strands separate2. Free-floating nucleotides pair with exposed bases3. Sugar-phosphate backbone bonds together

Two DNA molecules, each with one new and one old strand

1. Bond sugar and phosphate together2. Proofread new DNA strand

Check-in

What is the function of telomerase?

What is the major difference in DNA replication between prokaryotes and eukaryotes?

Add DNA sequences to tips of chromosomes to prevent genes from being lost or damaged during replication

In prokaryotic cells, replication starts in one spot on the chromosome, while in eukaryotic cells it starts in multiple spots.