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Chapter 25
Goals:
To be able to describe the structure of nucleosides, nucleotides and nucleic acids.
To understand the naming of nucleosides and nucleotides.
To be able to describe complementary base pairing in the DNA double helix.
To be able to describe how DNA replicates.
To be able to explain the process of protein synthesis via transcription and translation.
To understand the roles of mRNA, rRNA and tRNA in protein synthesis.
To be able to read the genetic code and determine the primary sequence of amino acids in a protein based
on the informational DNA, template DNA or mRNA structure.
  To understand how mutations in DNA can lead to genetic diseases.
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Nucleic Acids
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DNA: Deoxyribonucleic acid; found in cells (except for RBCs). Contains genetic information in segments called
genes. DNA is a “blueprint” for an organism. One gene codes for one protein.
RNA: Ribonucleic acid; also found in cells (except for RBCs). Responsible for transmitting and expressing the
genetic information of DNA by translation into proteins.
Structure of Nucleic Acids:
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Nucleoside:
–  A 5 membered cyclic sugar unit (ribose or deoxyribose) + nitrogen containing base (pyrimidine or purine
bases)
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Nucleotide:
–  A 5 membered cyclic sugar unit + nitrogen containing base + phosphate (a nucleoside and phosphate unit
linked via a phosphate ester bond)
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Nucleic Acids
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Naming nucleosides:
–  Prefix deoxy- if sugar is deoxyribose
–  No prefix if sugar is ribose
–  Base name –ending, +osine (for purines)
–  Base name –ending, +idine (for pyrimidines)
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Naming nucleotides:
–  Name nucleoside + monophosphate, diphosphate or triphosphate
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Nucleic Acids
•  Polynucleotides: The –OH of C-3 forms a phosphate diester linkage with the phosphate off of C-5. The free
phosphate group is the 5’ end; the free –OH group is the 3’ end.
•  Just as the amino acid order is important in proteins, the order of the nucleotides in nucleic acids is significant; 3’
TAG 5’ is not the same as 3’ GAT 5’!
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The Double Helix
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DNA has 2 polynucleotide strands that are held together by hydrogen bonds. The two strands are coiled together
to form a helical structure known as the double helix.
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The polynucleotide backbone is on the outside of the helix, and the bases are on the inside.
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Adenine is always opposite of thymine, and cytosine is always opposite of guanine. This is known as
complementary base pairing, and it is fundamental to your understanding of replication, transcription and
translation.
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Replication
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Replication: It is complementary base pairing that makes replication of DNA possible. The strands separate
into 2 polynucleotide chains. Each strand provides a blueprint for the other strand. From this blueprint, the other
(complementary) strand can be synthesized for two new daughter cells. Each blueprint strand is read in the 3’ to
5’ direction, and each new strand is replicated anti-parallel in the 5’ to 3’ direction.
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Transcription
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Transcription: The synthesis of RNA from DNA. Very similar to replication of DNA; short segments of DNA
unwind, and a short segment of RNA is synthesized from the DNA template. Once the RNA has been
synthesized, the DNA reforms the double helix. This occurs in the nucleus of cells.
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Translation
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Translation: The process by which the genetic message carried by RNA is decoded and used for protein
synthesis. Once synthesized, mRNA passes out of the nucleus to the cytoplasm of the cell where is meets up with
rRNA. tRNA is a polynucleotide that brings the appropriate amino acid to the sight of protein synthesis (rRNA).
When the correct mRNA meets up with the correct tRNA via complementary base pairing between the codon and
anticodon, amino acids form peptide bonds resulting in peptide/protein synthesis.
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Mutations and Genetic Diseases
Errors that are carried along during DNA replication are called mutations.
DNA
mRNA
AA
Properties of AA
Affect
CTC
GAG
Glu
Hydrophilic/Acidic
Normal
CTT
GAA
Glu
Hydrophilic/Acidic
None (Silent)
CTA
GAU
Asp
Hydrophilic/Acidic
None (Missense)
CAC
GUG
Val
Hydophobic/Neutral
Less water soluble (Missense)
ATC
UAG
Stop Codon
Not Applicable
No protein made/shortened protein
(Nonsense)
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Problems
1.  In transcription, does the RNA formed better resemble the informational strand or the template strand of DNA?
2.  Which of the following can have the sequence 3’ AUG CCA GUA 5’?
a.  DNA
b.  RNA
c.  Proteins
d.  Carbohydrates
e.  Fatty acids
3.  If a strand of informational DNA has the sequence 5’ ATG GCC CTG AAA 3’, what is the sequence of the template
DNA strand that runs antiparallel?
4.  What will be the sequence of the mRNA strand formed from the DNA informational strand shown in #3?
5.  What is the primary sequence for the peptide coded for by the DNA informational strand shown in #3?
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