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