Download DNA

Section 10 – 1 Objectives
• Explain the principal function of DNA.
• Describe the structure of DNA.
• Define the term complementary base
pairing.
• Explain the role of complementary base
pairing in the replication of DNA.
• Summarize the main features of DNA
replication.
Structure of DNA
•
Building blocks called nucleotides
1. Five carbon sugar molecule (pentose)
- deoxyribose
2. Phosphate group – P atom
surrounded by O atoms
3. Nitrogen Base
Nitrogen Bases
•
Purines - adenine(A) & guanine(G)
(1) contain 2 rings of carbon and nitrogen atoms
Nitrogen Bases
• Pyrimidines – cytosine(C) & thymine(T)
(1) contain 1 ring of carbon and nitrogen atoms
The Double Helix
1. Rosalind Franklin
and Maurice Wilkins
- provided X-ray
photographs of DNA
crystals
2. James Watson and
Francis Crick (1953)
- deciphered the
structure of DNA
The Double Helix
Complementary Base Pairing
1. Erwin Chargaff – determined that
%A = %T and %G = %C in any given
DNA sample
2. Chargaff’s Rule/Base Pairing Rules
a. Adenine pairs with thymine
b. Cytosine pairs with guanine
CBP
3. Bases are held together by hydrogen
bonds
a. There are 2 hydrogen bonds between
adenine and thymine.
b. There are 3 hydrogen bonds between
cytosine and guanine.
CBP
4. The complementary nucleotide chains in
the DNA model led to suggestion for how
DNA may copy itself.
Why must exact copies of DNA be passed
on to daughter cells?
Replication of DNA
Replication of DNA
1. Replication - is the process of copying
DNA in a cell
2. Replication fork - is the point at which the
two strands separate
- there may be thousands of these
along a single DNA molecule to permit
faster replication
Replication of DNA
3. Many enzymes are involved
a. DNA helicase – unwinds the DNA and
breaks the hydrogen bonds between
complementary bases
b. DNA polymerase – responsible for
adding new nucleotides to the growing
strand
DNA Replication
4. Replication is semi-conservative
- Each new DNA molecule consists of one
original strand and one new strand.
DNA Replication
5. Accuracy and Repair
•
a. Approximately one error in every 10,000
paired nucleotides
b. Enzymes proofread DNA and repair errors.
c. Mutation - a change in the DNA (nitrogen
base)
- caused by mutagens such as:
1. chemicals
2. radiation
3.viruses
Section 10-2 Objectives
• Explain the primary function of RNA
• Compare the structure of RNA with that of
DNA.
• Describe the structure and function of
each type of RNA.
• Summarize the process of transcription.
Structure of RNA
A. Building blocks called nucleotides.
DNA vs. RNA
DNA
RNA
Name of pentose
Nitrogen bases
# of strands
Location in cell
# of types
Types of RNA
1. messenger RNA (mRNA) – carries the
code from the nucleus to the cytoplasm
2. transfer RNA (tRNA) – carries amino
acids to the ribosome
3. ribosomal RNA (rRNA) – makes up part of
the ribosome
Transcription
A. Process by which genetic information is copied
from DNA to RNA
Steps of Transcription
1. Initiation
- The enzyme RNA polymerase
synthesizes RNA using the DNA as a
template.
- binds to regions of DNA called
promoters, which mark the beginning of
the DNA to be transcribed.
Transcription
2. Complementary base pairing
3. Termination
- sequence of 3 bases on DNA marks the
end of the gene
- RNA polymerase then releases the
RNA and the DNA molecule “rezips”
Products of Transcription
1. Called transcripts.
2. Include mRNA, tRNA, and rRNA.
3. Following transcription, mRNA leaves the
nucleus through the nuclear pores and
attaches to a ribosome where it will direct
protein synthesis
Section 10-3 Objectives
• Describe the genetic code.
• Distinguish between a codon and
anticodon and state where each is found.
• Explain the roles of the start codon and
stop codons.
• Summarize the process of translation.
Protein Structure and Composition
A Review of Proteins
1. Proteins are polymers of amino acids
that are linked together by peptide bonds.
2. There are 20 different amino acids.
3. The sequence of these amino acids
determines how the polypeptide will twist
and fold into the 3-D structure of a protein.
The Genetic Code
A. Codon
1. Sequence of 3 bases on mRNA that
codes for an amino acid.
2. There are 64 different codons.
3. Special Codons
a. The start codon is AUG
b. The stop codons are UAA, UGA,
and UAG.
Translation
• the process of assembling polypeptides
from information encoded of mRNA
tRNA and Anticodons
1. tRNA Structure
a. Amino acid
attachment site at the
top
b. anticodon - loop of
3 nucleotides at the
bottom
- this will pair with a
codon on the mRNA
Ribosomes
1. Composition
a. proteins
b. rRNA
Protein Assembly
1. Ribosome attaches to the start codon
(AUG) on the mRNA.
2. The tRNA with the anticodon (UAC) pairs
with AUG. This tRNA is carrying
the amino acid methionine.
3. The ribosome moves along the mRNA
transcript.
Protein Assembly
4. The next tRNA carrying its amino acid positions
itself beside the tRNA carrying methionine. A
peptide bonds forms between the two amino
acids.
5. The ribosome continues to move along the
mRNA while each mRNA codon is paired with a
tRNA anticodon.
6. A stop codon is reached, bringing an end to
translation.
7. Many ribosomes may simultaneously translate
the same mRNA, creating a structure called a
polyribosome.
Protein Assembly