Chapter 4: Nucleic Acids and the RNA World PDF

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Chapter 4: Nucleic Acids and the RNA World Nucleic Acids ↳ a polymer of nucleotide monomers Three Compounds of a nucleotide 1. A phosphate group 2. A five-carbon sugar 3. A nitrogenous (nitrogen containing) base Both the phosphate group and nitrogenous base are bonded to the sugar molecule

Ribonucleotides are the monomers of RNA ● The sugar is ribose ● Has an -OH group bonded to the 2’ carbon Deoxyribonucleotides are the monomers of DNA ● The sugar is deoxyribose (deoxy = lacking oxygen) ● Has an H instead at the 2’ carbon There are two groups of nitrogenous bases 1. Purines - contain nine atoms in their two rings a. Adenine (a) b. Guanine (G) 2. Pyrimidines - contain six atoms in their one ring a. Cytosine b. Uracil (U) - only in RNA c. Thymine (T) - only in DNA

Nucleotides Polymerize to Form Nucleic Acids ● Nucleic acids form when nucleotides polymerize via condensation reactions ● Phosphodiester linkage (bond) occurs between ○ The phosphate group on the 5′ carbon of one nucleotide – And the –OH group on the 3′ carbon of another ○ Two types of nucleotides are involved ● Ribonucleotides polymerize to form RNA ● Deoxyribonucleotides polymerize to form DNA

Structure of a Nucleic Acid - Phosphate Group

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Phosphate is bonded to the 5’ carbon, sugar is bonded to the 1’ carbon RNA transmits information, catalytic activity ○ Single strand helix ○ A,U,G,C DNA stores genetic info in modern cells ○ Double helix ○ A,T,G,C

RNA (like DNA) has a primary structure consisting of ● A sugar–phosphate backbone formed by phosphodiester linkages ● Four different types of nitrogenous bases extending from it ● The primary structure of RNA differs from DNA: 1. RNA contains ribose instead of deoxyribose• 2′ –OH group on ribose is more reactive than –H 2. RNA is much less stable than DNA 3. RNA contains the base uracil instead of thymine

DNA and RNA Strands Are Directional ● Phosphodiester linkages form a sugar–phosphate backbone ● Nucleic acids are directional ○ One end has an unlinked 5′ phosphate group ○ The other end has an unlinked 3′ hydroxyl group ● The order of nucleotides forms the primary structure ○ The sequence is written in the 5′ 3′ direction ○ Reflects the order that nucleotides are added to a growing molecule

DNA Strands Form an Antiparallel Double Helix ● James Watson and Francis Crick determined that

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two strands are held together by hydrogen bonds between pyrimidines and purines Complementary base pairing (Watson–Crick pairing) occurs between A and T, C and G DNA strands are antiparallel ■ One strand runs 3′5′, the other runs 5′3′ DNA strands form a double helix ■ The sugar–phosphate backbone faces the exterior ■ Nitrogenous base pairs face the interior

Hydrogen Bonding of Base Pairs ● G - C pairs have 3 hydrogen pairs ● A - T pairs have 2 hydrogen bonds DNA Strands Form an Antiparallel Double Helix ● One turn of the helix occurs every 10 base pairs ● Hydrophobic interactions in bases causes DNA to twist into a helix DNA Functions as an Information Molecule ● DNA can store and transmit biological information ● DNA carries the information required for the organism’s ● growth and reproduction ● The “language of nucleic acids” is contained in the sequence of the bases ○ Nitrogenous bases function like letters in an alphabet ○ The sequence of bases has meaning, like the order of letters in a word

DNA Functions as an Information Molecule ● DNA’s primary structure serves as a template for the synthesis of a complementary strand ○ Contains the information required for a copy of itself to be made ○ Requires enzymes for the process The Tertiary Structure of DNA ● DNA forms more compact three-dimensional structures in cells ○ When it is wound too tightly or loosely, it twists to form supercoils ○ It wraps around proteins-histones ● Compacting DNA ○ Allows discrete units for cell division ○ Helps DNA fit inside the nucleus ○ Contribute to its function The DNA Double Helix Is a Stable Structure ● DNA is very stable! ○ It is resistant to chemical degradation

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Makes it a reliable store for genetic information Covalent phosphodiester bonds important for stability

Stability of DNA is very important for life...