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What is the basic unit of DNA?
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The nucleotide, which is composed of deoxyribose (a sugar) bonded to both a phosphate group and a nitrogenous base
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What are the two types of bases?
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Purines, which are double-ringed, and the single-ringed pyrimidines
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What are the purines in DNA?
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Adenine and Guanine
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What are the pyrimidines?
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Cytosine and Thymine
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What do nucleotides bond together to form?
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Polynucleotides
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What is the 3’ hydroxyl group of the sugar on one nucleotide is joined to what?
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The 5’ hydroxyl group of the adjacent sugar by a phosphodiester bond
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How many bonds does T always form with A?
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Two hydrogen bonds
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G always forms how many hydrogen bonds with C?
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Three hydrogen bonds
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What does this base-pairing form?
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It forms rungs on the interior of the double helix that link the two polynucleotide chains together
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The strands are positioned how to each other?
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They are positioned antiparallel to each other
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What does this mean?
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It means one strand has a 5’ ‡ 3’ polarity, and its complementary strand has a 3’ ‡ 5’ polarity
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What is the 5’ end designated as?
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It is designated as the end with a free hydroxyl group bonded to the 5’ carbon of the terminal sugar
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What is the 3’ end designated as?
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The one with a free hydroxyl group attached to the 3’ carbon of the terminal sugar
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What is this known as?
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It is known as the Watson-Crick model of DNA
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What is semiconservative replication?
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During replication the helix unwinds and each strand acts as a template for complementary base-pairing in the synthesis of two new daughter helices
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What dos each daughter helix contain?
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It contains an intact strand from the parent helix and a newly synthesized strand
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What does this mean?
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It means DNA replication is semiconservative
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Where does replication begin?
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It begins at specific sites along the DNA called origins of replication and proceeds in both directions simultaneously
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What forms as replication proceeds in a given direction?
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A replication fork
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What does the enzyme helicase do?
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It unwinds the helix
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What does single-strand binding protein do?
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SSB binds to the single strands and stabilizes them, preventing them from recoiling and forming a double helix
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What does DNA gyrase do?
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It is a type of topoisomerase that enhances the action of helicase by the introduction of negative supercoils into the DNA molecule
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What is a primer chain?
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It is usually several nucleotides long and composed of RNA
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What does it do?
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It is necessary for the initiation of DNA synthesis
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What does the RNA polymerase, primase, do?
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It synthesizes the primer, which binds to a segment of DNA to which it is complementary and serves as the site for nucleotide addition
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What does the first nucleotide bind to?
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It binds to the 3’ end of the primer chain
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Which direction does DNA synthesis proceed in?
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It proceeds in the 5’ ‡ 3’ direction and is catalyzed by a group of enzymes collectively known as DNA polymerases
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What is the double-stranded DNA ahead of the DNA polymerase unwound by?
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A helicase, and SSB again keeps the unwound DNA in a single-stranded form so that both strands can serve as templates
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What does DNA gyrase do?
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It concurrently introduces negative supercoils to relieve the tension created during unwinding
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What is the leading strand?
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One of the daughter strands
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What is the other strand called?
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It is called the lagging strand
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What happens to the leading strand?
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It is continuously synthesized by DNA polymerase in the 5’ ‡ 3’ direction
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How is the lagging strand synthesized?
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It is synthesized discontinuously in the 5’ ‡ 3’ direction as a series of short segments known as Okazaki fragments
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What is the overall direction of growth of the lagging strand?
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It occurs in the 3’ ‡ 5’ direction
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What covalently links the fragments?
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DNA ligase
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How many strands is RNA usually?
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It is usually single stranded
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What are the several types of RNA?
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mRNA, tRNA, rRNA, and hnRNA
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What is messenger RNA?
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mRNA carries the complement of a DNA sequence and transports it from the nucleus to the ribosomes, where protein synthesis
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What is monocistronic?
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mRNA
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What does this mean?
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It means one mRNA strand codes for one polypeptide
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What is tRNA?
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It is found in the cytoplasm and aids in the translation of mRNA’s nucleotide code into a sequence of amino acids
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What does tRNA bring amino acids to?
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It brings them to the ribosomes during protein synthesis
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How many types of tRNA are there?
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Approximately one type of tRNA for each amino acid, so about 40
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What is rRNA?
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It is a structural component of ribosomes and is the most abundant of all RNA types
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Where is it synthesized?
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In the nucleolus
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What is hnRNA?
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It is a large ribonucleoprotein complex that is the precursor of mRNA
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What is transcription?
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It is the process whereby information coded in the base sequence of DNA is transcribed into a strand of mRNA
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What is mRNA synthesized from?
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A DNA template in a process similar to DNA replication
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Where does DNA helix unwind?
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It unwinds at the point of transcription, and synthesis occurs in the 5’ ‡ 3’ direction, using only one DNA strand as a template
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What is this template known as?
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The antisense strand
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