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Nucleic Acids and Protein
Synthesis
10 – 1 DNA
10 – 2 RNA
10 – 3 Protein Synthesis
DNA
• Ultimate source of genetic diversity is
Deoxyribonucleic Acid (DNA)
• Primary function of DNA
– Store and transmit the genetic information that
tells cells which proteins to make and when to
make them.
– Proteins in turn form the structural units of cells
and help control chemical processes within cells.
Structure of DNA
• DNA is an organic compound
• Made up of repeating subunits called
nucleotides
• Each DNA molecule consists of two long
chains of nucleotides
• Nucleotide made up of three parts
– Sugar molecule called Deoxyribose
– Phosphate group
– Nitrogen containing base
• Four different nitrogen bases
– Adenine - A
– Guanine - G
– Cytosine - C
– Thymine – T
• Purines
– Adenine and Guanine
* Pyrimidines
- Thymine and Cytosine
The Double Helix
• Discovered in 1953 by James Watson and
Francis Crick.
• Model said that DNA is composed of two
nucleotide chains that wrap around each
other to form a double spiral – Double Helix
• Model was inspired in part by X-ray
photographs of DNA crystals.
• The model provided an explanation for how
copies of DNA could be made and how genetic
information might be stored and used within
cells.
• Received Nobel Prize in 1962.
• Covalent bonds hold phosphate and sugar
together.
• Hydrogen bonds hold Nitrogen bases together.
• Form Twisted ladder.
• Phosphate and sugar make sides of ladder,
and nitrogen bases make rungs of ladder
• Rungs of ladder are all of uniform length.
• Each rung is made up of 1 purine and 1
pyrimidine. This makes sure every rung is the
same length.
• Double helix has a right handed twist
• Each full turn is made up of ten base pairs.
Complementary Base Pairs
• Adenine bonds with Thymine
• Guanine bonds with Cytosine
• Complementary base pairs led to suggestions
of how DNA might copy itself.
Replication of DNA
• Replication: process of copying DNA
• Step 1: Separation of two nucleotide chains.
• Point at which they separate: called
replication fork.
• Chains are separated by enzymes called
helicases, which break hydrogen bonds and
chains separate.
• Step 2: DNA polymerase bind to the
separated chains of DNA.
• New chains of DNA are assembled using
nucleotides in the surrounding medium that
are complementary to the existing DNA
chains.
• Step 3: Nucleotides are joined to the new
chains by covalent bonds between
deoxyribose and phosphate groups.
• They are joined to the original nucleotide
chain by hydrogen bonds.
• DNA replication does not begin at one end
and work its way to the other.
• DNA polymerase begin replication
simultaneously at many points along the
separated nucleotide chains.
• When replication is completed, two new exact
copies of the original DNA molecule are
produced.
Accuracy and Repair
• 1 error in every 10,000 paired nucleotides
• Mutation: a change in the nucleotide
sequence at even one location.
• Can have very serious effects in new cells.
• DNA proofreading and repair helps keep the
error rate to one error per 1 billion
nucleotides.
• Enzymes proofread and make repairs
RNA
• Nucleic Acid responsible for the movement of
genetic information form the DNA in the
nucleus to the site of protein synthesis in the
cytosol.
Structure of RNA
• A nucleic acid made up of repeating
nucleotides.
• Difference between DNA and RNA.
– Sugar molecule is Ribose
– No Thymine, Uracil instead
– Single strand vs. double strand
Types of RNA
• Three types of RNA
– Messenger RNA (mRNA) – consists of RNA nucleotides
in the form of a single uncoiled chain. mRNA carries
genetic information from the DNA in the nucleus to
the cytosol .
– Transfer RNA (tRNA) – consists of a single chain of
about 80 RNA nucleotides folded into a hairpin shape
that binds to specific amino acids.
– Ribosomal RNA (rRNA) – the most abundant form of
RNA. rRNA consist of RNA nucleotides in a globular
form. Joined by proteins, rRNA makes up the
ribosomes where proteins are made.
Transcription
• Process by which genetic information is copied
from DNA to RNA.
• Steps of transcription
– 1. RNA polymerase, enzyme, synthesizes RNA
copies of specific sequences of DNA
– RNA polymerase initiates RNA transcription by
binding to the specific regions of DNA called
promoters.
– Promoter marks the beginning of the DNA chain
that will be transcribed
• When RNA polymerase binds to a promoter,
the DNA molecule in that region separates.
• Only one of the separated DNA chains, called
the template is used for transcription
• RNA polymerase attaches to the first DNA
nucleotide of the template chain. Then it
begins adding complementary RNA
nucleotides to the newly forming RNA
molecule
• Transcription continues one nucleotide at a
time until the RNA polymerase reaches a DNA
region called the termination signal.
Products of Transcription
• Called transcripts
• mRNA, tRNA, rRNA
• mRNA moves through the pores of the nuclear
membrane into the cytosol of the cell, where
it will direct the synthesis of proteins.
Protein Synthesis
• The production of proteins.
• The amount and kind of proteins that are
produced in a cell determine the structure and
function of the cell.
• Proteins carry out the genetic instructions
encoded in an organisms DNA
Protein Structure and Composition
• Proteins are polymers
• Made up of one or more polypeptides, each of
which consists of a specific sequence of amino
acids linked together by peptide bonds.
• Polypeptides that make up one protein may
consist of hundreds or thousands of the 20
different amino acids determines how the
polypeptides will twist and fold into the three
dimensional structure of the protein.
• The function of a protein depends on its
ability to bind with other molecules within a
cell.
• The function depends on the protein’s three
dimensional structure, which is determined by
its amino acid sequence.
The Genetic Code
• The sequence of nucleotides in an mRNA
transcript is translated into a sequence of
amino acids.
• The genetic information necessary for making
proteins is encoded in series of three mRNA
nucleotides – called codon.
• Each codon codes for a specific amino acid.
Translation
• Process of assembling polypeptides from
information encoded in mRNA
• The process of translation begins when mRNA
leaves the nucleus through pores in the
nuclear membrane.
• The mRNA then migrates to a ribosome in the
cytosol, the site of protein synthesis
tRNA and Anticodons
• Amino acids from cytosol are transported to
the ribosomes by tRNA molecules.
• A tRNA molecule has a region that bonds to a
specific amino acid. The loop opposite the site
of amino acid attachment bears a sequence of
three nucleotides called an anticodon
• The tRNA anticodon is complementary to and
pairs with its corresponding mRNA codon.
Protein Assembly
• Amino acids are linked together by a peptide
bond (covalent)