Chapter 13

DNA, RNA, & Proteins

DNA

Deoxyribonucleic acid (DNA) is the

material that contains the information

that determines inherited characteristics.

In other words, it contains our genes.

The History of DNA

3 major experiments lead to the

conclusion that DNA is the genetic

material in cells. These

experiments were performed by

Griffith, Avery, Hershey, and Chase.

Griffith – Experimented on mice and

observed some harmless strains of

bacteria could change into harmful

strains. He called this transformation.

His study led to the conclusion that

genetic material could be transferred

between cells.

Transformation- a change in genotype

that is caused when cells take up

foreign genetic material.

Avery – 1940- wanted to determine if

the transforming agent in Griffith’s

experiments was protein, DNA, or

RNA. Discovered that DNA is the

nucleic acid that is responsible for

transformation.

He did this by using enzymes to

destroy each agent (one at a time)

and observing if R cells were

transformed into S cells.

Hershey-Chase experiment

Studied bacteriophages (viruses that

infect bacterial cells & cause the cells to

produce viruses)

Found that the genetic material of viruses

was DNA, not proteins

DNA Structure

DNA is a relatively simple molecule that is

composed of a 5 carbon sugar (deoxyribose),

a phosphate group, and 4 different subunits

known as nitrogenous bases:

-Adenine (A) -Guanine (G)

-Thymine (T) -Cytosine (C)

Gene- a segment of DNA that is

located in a chromosome and that

codes for a specific hereditary trait.

Chargaff’s Rules for base pairing

•Adenine always binds to Thymine A---T

•Guanine always binds to Cytosine G---C

•When two nitrogenous bases are bound

together, they are known as a base pair.

•These paired bases are called complimentary

because they fit together like puzzle pieces.

Purine- a nitrogenous base that

has a double ring structure (A & G)

Pyrimidine-a nitrogenous base that

has a single ring structure (T & C)

Watson & Crick

Built the first 3-D model of DNA using the

information from Chargaff, Rosalind Franklin

(x-rayed a DNA molecule), & their own

knowledge of chemical bonding.

Watson & Crick won a Nobel prize for their

discovery.

DNA Replication

1. Proteins called helicases separate the 2

original DNA strands

2. Complimentary nucleotides are added to

each strand by DNA polymerase

3. Two DNA molecules are formed that are

identical to the original DNA molecule

DNA replication- the process of

making a copy of DNA

DNA helicase- an enzyme that

unwinds the DNA double helix

during DNA replication

DNA polymerase- an enzyme that

catalyzes the formation of the DNA

molecule

DNA replication in prokaryotes

(binary fission)

1. Original Site of Replication Fork

2. Each of the Two Strands of DNA separate from each other and synthesis of a complementary strand on each parental strand begins

3. Loop of DNA Extending Out of The Plane of the Parental Bacterial Chromosome

4. Rotation Around Axis

BR - Copy the diagram and the question. Then,

answer the question.

Which sequence of

bases do the question

marks represent?

SWP – trans-?

EQ – Why is RNA needed to make proteins?

C C T A T G ? ? ?

G G A T A C C T G

DNA Replication Practice

•TTG GAG CGT GCT

•GCA CAT TTA CGA

•AAC GGC CTG CAG

Ribonucleic acid

RNA

RNA- a nucleic acid that is

essential in taking the genetic

information from DNA and building

proteins

3 types:

messenger RNA (mRNA)

transfer RNA (tRNA)

ribosomal RNA (rRNA)

mRNA

When DNA is transcribed into RNA, mRNA

is the type of RNA made. mRNA is

complementary to the DNA sequence of a

gene. The mRNA carries the instructions for

making a protein from a gene and delivers

them to the site of translation.

tRNA

During translation, tRNA “reads” the mRNA

sequence (codons) and builds amino acids

from those codes

rRNA

ribosomal RNA- RNA component of

ribosomes. Provides a mechanism for

decoding mRNA into amino acids & interacts

with tRNA during translation.

RNA Stucture:

-a single strand version of DNA

-contains 4 nitrogenous bases

(A,U,C,G)

-Uracil instead of Thymine

-A binds to U

-G binds to C

Differences in RNA & DNA RNA- Single stranded

sugar is ribose

uracil instead of thyamine

located in the nucleus & cytoplasm

DNA- double stranded

sugar is deoxyribose

thyamine instead of uracil

located in the nucleus

Transcription- the information in a

specific region of DNA (a gene) is

copied into mRNA

3 steps:

1. Initiation- RNA polymerase binds to

a DNA sequence in the gene known as

the promoter (the start point)

2. Elongation- The two DNA strands

unwind and separate, exposing the

bases. Complementary RNA nucleotides

are added by RNA polymerase, growing a

strand of mRNA.

3. Termination- The RNA polymerase

eventually reaches a ‘stop’ code at the

end of a set of genes. The mRNA

strand is released from the template

and the DNA strands close up and

reform the double helix.

The mRNA made in transcription

leaves the nucleus and takes its

sequence to the ribosome for

protein synthesis (translation).

Translation

Step 1: Initiation: mRNA is inserted into the small subunit

of a ribosome (at the 5l end). When the RNA encounters

the start sequence (AUG) , an initiator tRNA binds to the

ribosome.

Step 2: Elongation- tRNA reads the

codons (3 nucleotide sequence that

codes for amino acids), binding

anticodons to the mRNA strand &

linking amino acids together.

Step 3: Termination- The end of

translation occurs when the ribosome

reaches a STOP codon (UAA, UAG,

UGA). There are not any anticodons

for the STOP codons. The long chain

of amino acids (a polypeptide) is then

released from the ribosome.

http://upload.wikimedia.org/wikipedia/commons/b/b1/Ribosome_mRNA_translation_en.svg