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Chapter 5Part 5

Nucleic Acids

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• The amino acid sequence of a polypeptide is programmed by a discrete unit of inheritance known as a .• A gene is a segment of DNA that codes for a characteristic• DNA is an example of the polymer.• Nucleic acids are made of monomers of • There are 2 types of nucleic acids…• DNA and RNA

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•DNA = deoxyribonucleic acid•RNA = ribonucleic acid

• DNA and RNA are the molecules that enable living organisms to reproduce their complex components from generation to generation.• DNA provides directions for its own replication• DNA also directs RNA synthesis and through RNA,

controls protein synthesis.

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• Organisms inherit DNA from their parents.• Each DNA molecule is very long, carrying several

hundred or more genes.• Before a cell reproduces itself by dividing, its DNA is

copied. The copies are then passed to the next generation of cells.

• Although DNA encodes the information that programs all the cell’s activities, it is not directly involved in the day-to-day operations of the cell.• Each gene along a DNA molecule directs the synthesis of a

specific type of RNA called • The mRNA molecule interacts with the cell’s protein-

synthesizing machinery to direct the ordering of amino acids in a polypeptide.

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• The flow of genetic information is…• DNA RNA protein• This is called gene expression and consists of 2 processes called

transcription and translation.• Protein synthesis takes place on cellular structures called

ribosomes.• In eukaryotes, DNA is located in the nucleus, but most

ribosomes are located in the cytoplasm• mRNA functions as an intermediary, moving genetic instructions

for building proteins from the nucleus to the cytoplasm.• Prokaryotes lack nuclei but still use mRNA as intermediary

to carry a message from DNA to ribosomes.

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• Nucleotides…• They are the monomers of nucleic acids• They are called _____when organized into large

molecules.• Each nucleotide consists of…• Nitrogenous bases…• Rings of carbon and nitrogen• Are either purines or pyrimidines

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• Purines…• Pyrimidines…

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• 2 types of pentose sugar…• DNA = deoxyribose• RNA = ribose• Only difference between the 2 is the lack of an oxygen

atom on carbon 2 in deoxyribose.• Because the atoms in both the nitrogenous base and the

sugar are numbered, the sugar atoms are distinguished by a prime () after the number.• So the second carbon in the sugar ring is the 2 carbon

and the carbon that sticks up from the ring is the 5 carbon.

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• Polynucleotides are synthesized when adjacent nucleotides are joined by covalent bonds called _____________________________that form between the –OH group on the 3 of one nucleotide and the phosphate on the 5 carbon of the next.• This process creates a repeating backbone of sugar-phosphate

units, with appendages consisting of the nitrogenous bases.• The 2 free ends of the polymer are distinct.

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• The sequence of bases along a DNA or mRNA polymer is unique for each gene.• Because genes are normally hundreds to thousands of

nucleotides long, the number of possible base combinations is virtually limitless.

• The linear order of bases in a gene specifies the order of amino acids, the primary structure of a protein, which in turn determines the 3-D structure and function of a protein.

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• Nucleic Acid Structure…

• The sugar-phosphate backbones are on the outside of the helix (sides of the ladder)• The 2 backbones run in opposite 53 directions

from each other. This is called • The 2 polynucleotides or strands are held together by

hydrogen bonds between the 2 bases.• Most DNA molecules have thousands to millions of base

pairs.

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• Because of their shapes, only come bases are compatible with each other.• This is called the base-pairing rules… complementary base

pairs. • Before cell division, DNA must replicate.• Both strands serve as templates to make a new complementary

strand.• This results in 2 identical copies of the original double-stranded

DNA molecule that then get distributed to the daughter cell.• This mechanism ensures that a full set of genetic information is

transmitted when a cell reproduces.

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• Complementary base pairing also occurs between parts of 2 RNA molecules or even between 2 stretches of nucleotides in the same RNA molecule.• Transfer RNA or tRNA brings amino acids to the ribosomes during

polypeptide synthesis.• The functional shape of tRNA results from base-pairing between

nucleotides where complementary stretches of the molecule run antiparallel to each other.

• In RNA adenine pairs with uracil. No thymine present.• RNA molecules can exist in various shapes while DNA is

always a helix.

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