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METABOLISM, THE SYNTHESIS OF NUCLEIC
ACIDS AND PROTEINS
I.
Nucleic Acid Structure:
A.
Nucleic acids:
Bipolymeres:
Consist of:
Pentose phosphate chains
Nitrogenous bases:
Two types of nitrogenous bases:
Purines:
Adenine
Guanine
Pyrimidines:
Thymine (DNA only)
Cytosine
Uracil (RNA only)
Form linkages with pentose sugars:
Nucleoside:
Base + 5 Carbon sugar
Nucleotide:
Base + 5 Carbon sugar + PO4:
Phosphate group (PO4):
Combines with two different five carbon
sugars:
B.
Forms backbone of nucleic acid molecule:
DNA Structure:
Long molecules
Composed of two polynucleotide chains
Coiled together - double helix
Backbone:
Alternating deoxyribose and PO4 groups:
Held together by hydrogen bonding between bases:
Strands are complementary:
Adenine always pairs with Thymine:
Two hydrogen bonds
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Guanine always pairs with Cytosine:
Three hydrogen bonds
Differences between specific DNA molecules due to difference in base
sequence
C.
RNA STRUCTURE:
Long molecules:
Single stranded molecule:
Contains:
Five carbon sugar:
Ribose
Pyrimidine:
Uracil
Can coil back on itself:
Base pairing occurs:
Forms hair-pin shaped structures
Helical organization
Three kinds:
Messenger RNA (mRNA)
Transfer RNA (tRNA)
Ribosomal RNA (rRNA)
II.
Organization of DNA in cells:
A.
Prokaryotic cells:
Closed circle:
Twisted into supercoiled DNA
Associated with basic proteins:
Not associated with histones
Basic proteins:
Organized bacterial DNA into coiled chromatin-like structure
B.
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Eukaryotic cells:
More highly organized:
Linear:
Associated with histones:
Small basic proteins rich in:
Lysine
Arginine
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Arranged in nucleosomes
III.
DNA Replication
A.
Replication pattern varies between prokaryotic and eukaryotic cells:
B.
General steps
Two strands of double helix:
Unwind
Separate - helicase
Free nucleotides move in
Line up along the two parental strands via base paring
Linked together by DNA polymerases:
750 -1000 bases/second
IV
RNA transcription:
A.
DNA not direct template for protein sequence:
Transfers information to RNA
RNA is transcribed from DNA
B.
All RNA molecules have same backbone structure:
Ribose alternating with phosphate:
C.
Differences in RNA molecules:
Due to the differences in the order of bases connected to the sugar
phosphate backbone:
Any possible sequence of:
Adenine
Uracil
Cytosine
Guanine
D.
Single DNA strand is template for RNA synthesis:
Base sequence of RNA complementary to base sequence of DNA
template:
E.
RNA bases form H bonds with bases of DNA molecule:
Ribose sugar phosphate backbone zipped up by RNA polymerase
RNA polymerase must bind to promoter before transcription can start
Promoter:
Region in DNA where RNA polymerase must bind before it can
make the linkage between ribonucleotides and phosphate
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F.
V.
Messenger RNA (m RNA):
Directs amino acid sequence in proteins
Template for protein synthesis
Transcribed from a specific gene which codes for a specific protein:
Specific gene contains:
Series of codons:
Sequence of three bases coding for an single amino acid:
Genetic code
After transcription:
mRNA associates with a ribosome
Protein Synthesis:
A.
Translation:
Synthesis of protein using an RNA template
Amino acid activation:
Amino acid attached to transfer RNA:
tRNA = Adaptor molecule:
Modifies amino acids so that they can combine with a
mRNA codon
Small - 60 - 70 nucleotides long
Contain anticodon:
Three bases complementary to the codon in RNA
At least one specific tRNA for each amino acid
Amino Acetyl tRNA Synthetase:
Links amino acids to tRNAs
Specific enzyme for each amino acid
Occurs only on surface of ribosomes
Ribosome:
Orients the AA-tRNA combination and mRNA so that
genetic code can be read:
Have specific surfaces that bind mRNA and tRNA
B.
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Prokaryotic ribosome
Composed of 2 separate subunits:
Subunits separate when not actively synthesizing protein
30s unit contains:
21 different kinds of protein
16s RNA
50s unit contains:
5
32-34 kinds of protein
RNA:
23s RNA
5s RNA
Combined units = 70s
C.
Eukaryotic ribosome
Composed of 2 separate subunits:
Subunits separate when not actively synthesizing protein:
40s unit contains:
32 different kinds of protein
18s RNA
60s unit contains:
40 kinds of protein
RNA:
28s RNA
5s RNA
Combined units = 80s
D.
Initiation of protein synthesis requires:
Initiation complex:
Consists of:
30s ribosome
mRNA
N-formylmethionyl-tRNA
Must be formed before protein synthesis starts:
Combines with 50s subunit to form functional ribosome
Initiation of protein synthesis is complex:
Insures that protein synthesis starts at the correct location:
At the beginning of a gene
At the correct codon
At the correct base in the codon
E.
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Elongation of the polypeptide Chain:
50s subunit has two special sites involves in the elongation of the polypeptide
chain:
Acceptor (A) site:
Place where new tRNA-aa attach to 50s subunit
Peptidyl (P) site:
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Place where growing polypeptide chain held by a tRNA
Peptidyl transferase:
Part of the 50s ribosome
Makes peptide bond between adjacent amino acids
F.
Protein synthesis continues until a termination (nonsense) codon is reached:
Termination Codons:
UAA
UAG
UGA
Code for no amino acid
Stop polypeptide elongation
G.
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Protein synthesis requires more energy than any other synthetic process in the cell:
Four ATP molecules per peptide bond