CHEM 1406 Concept Review: Nucleic Acids and Protein Synthesis

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Components of Nucleic Acids

Nucleotides are the fundamental building block of nucleic acids.  Each nucleotide is composed of the following:   1) A phosphate group 2) A 5-carbon sugar (either ribose for RNA or deoxyribose for DNA) 3) A nitrogenous base. There are only 5 bases that make up nucleic acids.  Adenine, Guanine, and Cytosine are common to both DNA and RNA.  Thymine is only present in DNA, and Uracil is only present in RNA.   Nucleosides refer to just the sugar molecule bound to the nitrogenous base. (without the phosphate group)

Image 17.1: Components of a Nucleotide

 

Table 17.1:  Naming Nucleosides and Nucleotides in DNA and RNA

Base	Name of Nucleoside	Name of Nucleotide
For DNA
Adenine (A)	Deoxyadenosine (A)	Deoxyadenosine-5’-monophosphate (dAMP)
Guanine (G)	Deoxyguanosine (G)	Deoxyguanosine-5’-monophosphate (dGMP)
Cytosine (C)	Deoxycytidine (C)	Deoxycytidine-5’-monophosphate (dCMP)
Thymine (T)	Deoxythymidine (T)	Deoxythymidine-5’-monophosphate (dTMP)
For RNA
Adenine (A)	Adenosine (A)	Adenosine-5’-monophosphate (AMP)
Guanine (G)	Guanosine (G)	Guanosine-5’-monophosphate (GMP)
Cytosine (C)	Cytidine (C)	Cytidine-5’-monophosphate (CMP)
Uracil (U)	Uridine (U)	Uridine-5’-monophosphate (UMP)

 

Complementary Base Pairs: DNA and RNA base pairs will match up in such a way as to maximize hydrogen bonding and optimize distance between complementary strands.  As such, a two-membered ring (purine) will always be paired with a one-membered ring (pyrimidine).  More specifically, the base pairs that match each other are as follows:

 

 

 

The Primary Structure of DNA

Image 17.2: Molecular Structure of DNA

As seen above, nucleotides connect “vertically” with phosphodiester bonds, where the phosphate group is connected to the 5’ carbon of one sugar molecule AND the 3’ carbon of a different sugar molecule. The bond angles and spatial arrangements of these groups lead to double-stranded DNA being twisted into the famous double helix shape we are all so familiar with.

 

 

DNA Replication

Image 17.3: Step Involved in DNA Replication (Goes with Table 17.2)

 

Table 17.2:  Steps Involved in DNA Replication

Leading Strand		Lagging Strand
Step 1: DNA Helicase catalyzes the unwinding of the double helix by breaking the hydrogen bonds between complementary base pairs.		Step 1: DNA Helicase catalyzes the unwinding of the double helix by breaking the hydrogen bonds between complementary base pairs.
Step 2: Single-strand binding proteins stabilize the unwound parental DNA.		Step 2: Single-strand binding proteins stabilize the unwound parental DNA.
Step 3a: DNA polymerase catalyzes the formation of phosphodiester bonds between incoming free nucleotides that match up with complementary bases on the parental DNA strand continuously until the new daughter DNA molecule is complete.		Step 3b: RNA primase places an RNA primer on the lagging parental DNA strand to give a starting point for DNA polymerase to begin acting.
		Step 4b: DNA polymerase catalyzes the formation of phosphodiester bonds between incoming free nucleotides that match up with complementary bases on the parental DNA strand from the previous RNA primer to the next RNA primer.
		Step 5b: After the replacement of the RNA primers by a different version of the DNA polymerase enzyme, DNA ligase links together the different (Okazaki) fragments to complete the synthesis of the new daughter DNA molecule from the lagging strand.

 

 

RNA and the Genetic Code

RNA makes up most of the nucleic acid in a cell.  DNA and RNA are both nucleotide polymers.  Several important differences between RNA and DNA are as follows:

1) In RNA, the sugar that helps make up the backbone of RNA is ribose, as opposed to deoxyribose in DNA.

2) In RNA, the nitrogenous base that pairs with adenine is uracil, whereas thymine pairs with adenine in DNA.

3) RNA molecules are usually single stranded, whereas DNA molecules are double stranded.

4) RNA molecules (polymers) are much smaller than DNA molecules (polymers).

5) RNA molecules are the “actors” in the cells: synthesizing proteins, carrying information, transporting amino acids, and even acting as “enzymes” to catalyze reactions.

Table 17.3:  Types and Functions of RNA Molecules in Humans

Type	Abbreviation	Function in the Cell	% of Total RNA
Ribosomal RNA	rRNA	Synthesizes proteins; major component of ribosomes	~80%
Messenger RNA	mRNA	Carries information for protein synthesis from the DNA in the nucleus to the ribosomes	~5%
Transfer RNA	tRNA	Brings amino acids to the ribosomes for protein synthesis	~15%
Catalytic RNA (ribozymes)	none	Catalyze a number of important reactions, primarily those involving modification of RNA	less than 1%

*At least 4 other additional forms of RNA have been discovered that have their own unique and important functions in cells.

 

Transcription: the synthesis of mRNA from DNA.  In this process, a relatively small part of DNA that codes for a gene is used to synthesize mRNA, which is then sent to the ribosomes for protein synthesis.

Translation: the synthesis of protein from mRNA using the cellular machinery of ribosomes and tRNA

Codon: a three nucleotide sequence that codes for a specific amino acid

 

 

Genetic Mutation: a change in the nucleotide sequence of DNA.  Such changes may alter the sequence of amino acids in a protein, affecting its structure and function.

Frameshift Mutation: one or a few bases are added or deleted from the normal order of the nucleotide bases in the template strand of DNA.  These mutations can be particularly catastrophic because they change the entire sequence of amino acids.

Nucleotide Substitution: also called a point mutation, this type of mutation involves the replacement of exactly one nucleotide with a different one in the template strand of DNA.  When translated into mRNA, this sequence will then change the codon for one amino acid. When this change occurs, it may (but does not always) change one amino acid in the sequence of a protein.  However, even a change of one amino acid can have disastrous effects on protein structure and function.  (Sickle cell anemia is an example of this type of substitution.)