DNA Structure and Function

DNA Structure• DNA is a macromolecule that stores

and transfers information in living cells.

• It is found in the nucleus of eukaryotic cells and in the cytoplasm of prokaryotic cells.

• It is also found in mitochondria and chloroplasts.

DNA Structure

• DNA consists of long strands of nucleotides.

• Each nucleotide consists of

– Deoxyribose (5-carbon sugar)

– Phosphate group

– A nitrogen-containing base

• Four bases

– Adenine, Guanine, Thymine, Cytosine

Nucleotide Bases

phosphate group

deoxyribose

ADENINE (A)

THYMINE (T)

CYTOSINE (C)

GUANINE (G)

Composition of DNA

• Chargaff showed:

– Amount of adenine relative to guanine

differs among species

– Amount of adenine always equals amount

of thymine and amount of guanine always

equals amount of cytosine

A=T and G=C

Rosalind Franklin’s Work

• Was an expert in x-ray crystallography

• Used this technique to examine DNA fibers

• Concluded that DNA was some sort of helix

Structure of the Hereditary Material

• Experiments in the 1950s showed that DNA is the hereditary material

• Scientists raced to determine the structure of DNA

• 1953 - Watson and Crick proposed that DNA is a double helix

• http://nobelprize.org/educational_games/medicine/dna_double_helix/

Watson-Crick Model

• DNA consists of two nucleotide strands

• Strands run in opposite directions

• Strands are held together by hydrogen

bonds between bases

• A binds with T and C with G

• Molecule is a double helix

Watson-Crick Model

http://learn.genetics.utah.edu/units/basics/builddna/

DNA Function

• DNA can copy itself - REPLICATION

• DNA transfers information to new strands of DNA.

• DNA can transfer its information to RNA - TRANSCRIPTION

• RNA then translates information into an amino acid sequence - TRANSLATION

DNA Structure Helps Explain How it Duplicates

• DNA is two nucleotide strands held

together by hydrogen bonds

• Hydrogen bonds between two strands

are easily broken

• Each single strand then serves as

template for new strand

DNA Replication

newnew old old

• Each parent strand

remains intact

• Every DNA

molecule is half

“old” and half “new”

Base Pairing During

Replication

Each old strand serves as the template for complementary new strand

Enzymes in Replication

• DNA helicase unwind the two strands

• DNA polymerase attaches

complementary nucleotides

• DNA ligase fills in gaps

• Other enzymes wind two strands

together.

A Closer Look at Strand Assembly

Energy for strand assembly is provided by removal of two phosphate groups from free nucleotides

newlyformingDNAstrand

one parent DNA strand

Continuous and Discontinuous Assembly

Strands can only be assembled in the 5’ to 3’ direction. This refers to the number of the carbon of the sugar

DNA Repair

• Mistakes can occur during replication

• DNA polymerase can read correct

sequence from complementary strand

and, together with DNA ligase, can

repair mistakes in incorrect strand

Same two steps produce ALL proteins:

1) DNA is transcribed to form RNA– Occurs in the nucleus– RNA moves into cytoplasm

2) RNA is translated to form polypeptide chains, which fold to form proteins

Steps from DNA to Proteins

Three Classes of RNAs

• Messenger RNA

– Carries protein-building instruction

• Ribosomal RNA

– Major component of ribosomes

• Transfer RNA

– Delivers amino acids to ribosomes

A Nucleotide Subunit of RNA

phosphate group

sugar (ribose)

uracil (base)

Base Pairing During Transcription

• A new RNA strand can be put together

on a DNA region according to base-

pairing rules

• As in DNA, C pairs with G

• Uracil (U) pairs with adenine (A)

Transcription & DNA Replication

• Like DNA replication– Nucleotides added in 5’ to 3’ direction

• Unlike DNA replication– Only small stretch is template

– RNA polymerase catalyzes nucleotide addition

– Product is a single strand of RNA

Promoter• A base sequence in the DNA that signals the

start of a gene

• For transcription to occur, RNA polymerase must first bind to a promoter

transcribed DNA winds up again

DNA to be transcribed unwinds

mRNAtranscript

RNA polymerase

Adding Nucleotides

growing RNA transcript5’

3’5’

3’

direction of transcription

Transcript Modificationunit of transcription in a DNA strand

exon intron

mature mRNA transcript

poly-A tail

5’

5’ 3’

3’

snipped out

snipped out

exon exonintron

cap

transcription into pre-mRNA

3’ 5’

Genetic Code

• Set of 64 base triplets – ex: AGA CCC

• Codons - Nucleotide bases read in blocks of three

• 61 of the base triplets specify amino acids

• 3 stop codons – UGA, UAA, UAG

• http://nobelprize.org/educational_games/medicine/gene-code/how.html

Genetic Code

Genetic Code

Code Is Redundant

• Twenty kinds of amino acids are

specified by 61 codons

• Most amino acids can be specified by

more than one codon

• Six codons specify leucine

– UUA, UUG, CUU, CUC, CUA, CUG

tRNA Structure

codon in mRNA

anticodon in tRNA

amino acid OH

tRNA molecule’s attachment site for amino acid

Ribosomestunnel

small ribosomal subunit large ribosomal subunit intact ribosome