BIOLOGY 11
Mrs. Garrison
NUCLEIC ACIDS AND PROTEIN SYNTHESIS
Nucleic Acids
Deoxyribonucleic acid (DNA)
Ribonucleic acid (RNA)
Both Composed of nucleotides
Sugar + Phosphate + bases
Deoxyribonucleic acid (DNA)
Deoxyribose (sugar)
Phosphate
Bases
Adenine
Guanine
Cytosine
Thymine
Double-stranded alpha-helix
Sugar and phosphate alternate to form "rails" of ladder
Bases pair between the "rails" to form "rungs"
Adenine pairs with thymine
Guanine pairs with cytosine
Carries hereditary information
Chromosomes made of chromatin (=DNA + histone proteins)
DNA replicates itself prior to cell division
Need exact copy of each double-stranded DNA molecule
Enzymes required at each step
Double strands of DNA separate
Each strand acts as template on which a new complementary strand is
made by pairing the bases of new nucleotides with the bases of the
template DNA strand
New complementary strand is identical to old strand paired with
template
Have 2 identical double-stranded DNA molecules
Each is half old DNA and half new DNA
Synthesis of complementary strand occurs from 5' to 3' direction
Mistakes corrected for by enzymes
Ribonucleic Acid (RNA)
Single stranded molecule from which protein is built
Composed of nucleotides
Ribose (sugar)
Phosphate
Bases
Adenine
Guanine
Cytosine
Uracil (no thymine)
RNA synthesis called transcription
RNA is built from a segment of DNA called a gene
Requires enzymes and energy
Occurs in nucleus on uncoiled DNA
Double DNA strands separate
One strand acts as template
New nucleotides are added by base pairing to build a
complementary RNA strand from the DNA template, starting at
promoter site on DNA
Guanine pairs with cytosine of DNA
Cytosine pairs with guanine of DNA
Adenine pairs with thymine of DNA
Uracil pairs with adenine of DNA
RNA strand pulls away from DNA and DNA strands again pair up
RNA strand is modified
Non-coding portions (introns) cut out
Coding portions (exons) spliced together
RNA strand leaves nucleus through pores in nuclear membrane
Three types of RNA
Messenger RNA (mRNA) - codes for amino acids of protein
Ribosomal RNA (rRNA) - combines with enzymes to form 2 ribosomal
subunits, which are location of protein synthesis
Transfer RNA (tRNA) - brings amino acids to ribosome
Proteins
Protein synthesis is called translation
Genetic code
Information coded for by the base sequence of mRNA (which was coded for by
the base sequence of DNA) is used to build a sequence of amino acids,
forming a protein
mRNA bases arranged in codons of 3 bases each
tRNA has a triplet anticodon which pairs with mRNA codon
Each tRNA carries specific amino acid, depending on the anticodon
Each amino acid has 2 to 4 different codons which code for it
Initiation of translation
Small ribosomal unit binds to mRNA at start codon (AUG)
Initiator tRNA brings in first amino acid
Large ribosomal subunit binds to complex
Chain elongation
As each succeeding mRNA codon is moves into the binding site on ribosome,
the tRNA with the proper anticodon comes in with its amino acid
Each successive amino acid is bound to the previous amino acid by a
peptide bond and the previous tRNA releases its amino acid and leaves
Chain termination
Occurs when a stop codon (UAG, UAA, UGA) is reached
Polypeptide chain released to cytoplasm or ER (if modification required)
Mistakes
Insertions or deletions of DNA nucleotides result in incorrect mRNA codons
mRNA codons following the mistake will be misread
Three insertions or deletions will bring the codons back into the correct
reading frame
Replacement of correct nucleotide with incorrect nucleotide will result in
just one mRNA codon being wrong
The first two nucleotides of codon most important,a mistake causing the
third nucleotide to be wrong will often cause no problem
Mistakes in DNA will therefore cause mistakes in mRNA codons, which will
cause mistakes in amino acid sequence (primary structure) of protein
Mistakes in primary structure may result in mistakes in tertiary structure
and a non-functioning protein