Ch3 Nucleic Acids

Deoxyribonucleic acid (DNA)

stores the information and passes it to the next generation

Ribonucleic acid (RNA)

carries that information to where proteins get made

A gene

is a stretch of DNA that specifies the amino acid sequence of a polypeptide

THE FLOW OF INFORMATION STEP ONE

DNA in the nucleus is used to synthesize mRNA — that is transcription

THE FLOW OF INFORMATION STEP TWO

the mRNA moves out of the nucleus into the cytoplasm

THE FLOW OF INFORMATION STEP THREE

a ribosome reads the mRNA and builds a polypeptide — that is translation

the central dogma

DNA → RNA → protein.

Nucleic acids are polymers

of nucleotides

Each nucleotide has exactly three parts:

A nitrogenous base, A five-carbon sugar (a pentose), One or more phosphate groups

Base plus sugar alone is a

nucleoside

Pyrimidines

have one six-membered ring: cytosine, thymine (DNA only) and

uracil (RNA only)

Purines

are larger, with a six-membered ring fused to a five-membered ring:

adenine and guanine

Memory hook

PURe As Gold — PURines are A and G. Everything else is a pyrimidine

SIZE MATTERS

a purine always pairs with a pyrimidine.

Ribose

is the sugar in RNA

Deoxyribose

is the sugar in DNA — "deoxy" because it is missing one oxygen, at the 2′ carbon

a single missing oxygen makes ___ chemically more stable than

DNA, RNA

Nucleotides join by dehydration

the same reaction as every other polymer this week

The phosphate of ___ bonds to the sugar of the next, creating a ___

one nucleotide, sugar-phosphate backbone

BUILDING THE POLYMER:The strand has direction

a 5′ end and a 3′ end, named for the carbons of the sugar

THE DOUBLE HELIX

Watson and Crick, 1953, building on Rosalind Franklin’s X-ray diffraction data

DNA is two

polynucleotide strands wound around each other

The sugar-phosphate backbones run on the outside

the bases point inward

DOUBLE HELIX: The two strands are antiparallel

one runs 5′ to 3′, the other 3′ to 5′

___ between the bases hold the two strands together.

Hydrogen bonds

Adenine always pairs with

thymine

Guanine always pairs with

cytosine

A–T is held by

two hydrogen bonds

G–C is held by

three hydrogen bonds

A purine always pairs with a ___, which keeps the helix a uniform width

pyrimidine

BASE PAIRING IS THE WHOLE TRICK: And here is the consequence

the two strands are complementary. Each one carries the information needed to rebuild the other — which is how DNA gets copied

RNA is usually

single-stranded

But a ___ can fold back on itself, base-pairing in places, and take on a specific three- dimensional shape

single strand

Transfer RNA is the classic example

it folds into a shape that lets it deliver amino acids to the ribosome

Notice the parallel to proteins

sequence determines folding, and folding determines function.

Same principle, different molecule

Sugar: deoxyribose in

DNA

Sugar: ribose

RNA

Bases: A, T, G, C

in DNA

Bases: A,U,G,C

in RNA — uracil replaces thymine

Strands: double in

DNA

Strands: usually single in

RNA

Job: long-term storage in

DNA

Job: carrying and interpreting the message in

RNA

DNA

is inherited

Carbohydrates

monosaccharides → glycosidic linkages → fuel and structure

Lipids:

not polymers → hydrophobic → storage, membranes, signals

Proteins:

amino acids → peptide bonds → almost every job in the cell

nucleic acids:

nucleotides → phosphodiester backbone → information

Nucleotides have three parts:

nitrogenous base, five-carbon sugar, phosphate

DNA is

a double helix with antiparallel strands, A–T and G–C base pairing, and complementary strands

RNA

s single-stranded, uses uracil and ribose, and folds into functional shapes