molecular biology of the gene dna structure and function
TRANSCRIPT
Molecular Biology of the GeneDNA Structure and Function
History of DNA• 1869 Johann Friedrich Miescher
• 1924 Microscope studies using stains for DNA and protein show that both substances are present in chromosomes.
• 1952 Alfred Hershey and Martha Chase
SCIENTIFIC DISCOVERY: DNA is a double-stranded helix
Erwin Chargaff
Rosalind Franklin / Maurice Wilkins
James Watson and Francis Crick
In 1962, the Nobel Prize
SCIENTIFIC DISCOVERY: Experiments showed that DNA is the genetic material
Until the 1940s, the case for proteins serving as the genetic material was stronger than the case for DNA.
– Proteins are made from ____different amino acids.
– DNA was known to be made from just ____ kinds of nucleotides.
Studies of bacteria and viruses
– ushered in the field of molecular biology, the study of heredity at the molecular level, and
– revealed the role of DNA in heredity.
DNA and RNA are polymers of nucleotides
DNA and RNA are nucleic acids.
The building blocks or monomers of nucleic acids are ____________________
A nucleotide is composed of a
– _________________
– _________________
– __________________
The nucleotides are joined to one another by a bond creating the sugar-phosphate backbone.
A
A
A
A
A
A
A
C
T
T
T
T
T
T
C
C
C
C
G
G
G
G
G
C
C G
AT
A DNAdouble helix
T
DNAnucleotide
Covalentbondjoiningnucleotides
A
C
T
Two representationsof a DNA polynucleotide
G
G
G
G
C
T
Phosphategroup
Sugar(deoxyribose)
DNA nucleotide
Thymine (T)
Nitrogenous base(can be A, G, C, or T)
Sugar
Nitrogenousbase
Phosphategroup
Sugar-phosphatebackbone
4 Different Types of Nucleotides Found in DNA
Base pair
Hydrogen bond
Partial chemicalstructure
Computermodel
Ribbonmodel
3’ and 5’ ends of nucleotide strand
Purine or pyrimidine ?
Hydrogen bonds hold the 2 strands together
2 General Functions for DNA
1.
2.
DNA replication depends on specific base pairing
In their description of the structure of DNA, Watson and Crick noted that the structure of DNA suggests a possible copying mechanism.
DNA replication follows a semiconservative model.
DNA Replication
SEMICONSERVATIVE
1. Helix unwinds
2. 2 strands separate
3. Free nucleotides bind to open bases according to pairing rules
(parent strand acts as template)
4. 2 identical strands consist of one parent strand and one newly formed strand.
DNA replication begins at the origins of replication where
– DNA unwinds at the origin to produce a “bubble,”
– replication proceeds in both directions from the origin, and
– replication ends when products from the bubbles merge with each other.
DNA replication occurs in the 5 to 3 direction.– Replication is continuous on the 3 to 5 template.– Replication is discontinuous on the 5 to 3 template,
forming short segments.
DNA replication proceeds in two directions at many sites simultaneously
Leading and Lagging strands
Why? DNA polymerases can only assemble new strands in the 5’-> 3’ direction, need a 3’ end
(-OH) provided by RNA primer.
Overall direction of replication
DNA ligase
Replication fork
Parental DNA
DNA polymerasemolecule This daughter
strand is synthesizedcontinuously
This daughterstrand is synthesizedin pieces
35
35
3
5
35
DNA replication proceeds in two directions at many sites simultaneously
Key proteins are involved in DNA replication.
– Helicase
– DNA Polymerases-
– Primase
– Proofreader
– DNA ligase
ParentalDNAmolecule Origin of
replication
“Bubble”
Parental strand
Daughter strand
TwodaughterDNAmolecules
http://207.207.4.198/pub/flash/24/menu.swf
ANIMATION…Replication
http://highered.mcgraw-hill.com/olc/dl/120076/bio23.swf
http://www.phschool.com/science/biology_place/biocoach/dnarep/intro.html
DNA Repair
DNA processes
• Replication
• Protein Synthesis
–Transcription
–Translation
Protein Functions….
• Metabolism (enzymes are proteins)
• Structural (build form)
• Transport (ex- hemoglobin)
• Protection (antibodies are proteins)
• Cell communication (hormones)
Review of Protein Structure…
Only 20 different common amino acids
Structure determines function !
Hundreds of thousands of different proteins !
The DNA genotype is expressed as proteins, which provide the molecular basis for
phenotypic traits
DNA specifies traits by dictating protein synthesis.
The molecular chain of command is from
– DNA in the nucleus to RNA and
– RNA in the cytoplasm to protein.
__________________ is the synthesis of RNA under the direction of DNA.
__________________ is the synthesis of proteins under the direction of RNA.
3 parts of RNA (the other nucleic acid) nucleotide
sugar = _________
phosphate group
nitrogenous base (A,U,C,G)
U=Uracil
Nucleotides: 2 types DNA & RNA
3 Types of RNA Required for Protein Synthesis
• mRNA= messenger RNA
• tRNA= transfer RNA
• rRNA= ribosomal RNA
DNA
NUCLEUS
CYTOPLASM
RNA
Transcription
Translation
Protein
T
Strand to be transcribed
A C T T C AA
A A A T
DNAAA T C
T T T T G A G G
RNA
Transcription
A A A A U U U U U G G G
Translation
Polypeptide Met Lys Phe
Stopcodon
Startcodon
The DNA genotype is expressed as proteins, which provide the molecular basis for
phenotypic traits
The connections between genes and proteins
– The initial one gene–one enzyme hypothesis was based on studies of inherited metabolic diseases.
– The one gene–one enzyme hypothesis was expanded to include all proteins.
– Most recently, the one gene–one polypeptide hypothesis recognizes that some proteins are composed of multiple polypeptides.
Transcription- overview
Transcribing (writing) information from DNA
Takes place in the nucleus
Promoter region is recognized by RNA polymerase as a start location
Assembles mRNA strand
mRNA
RNApolymerase
Free RNAnucleotides
Templatestrand of DNA
Newly made RNA
Direction oftranscription
TG
AG G
A
A
U C C AC
T TA
A
CC
GGU
T UTAACCT
A
TC
TRANSCRIPTION
RNA polymerase
DNA of gene
PromoterDNA
Initiation1
2
TerminatorDNA
3
Elongation Area shownin Figure 10.9A
TerminationGrowingRNA
RNApolymerase
CompletedRNA
Genetic information written in codons is translated into amino acid sequences
The sequence of nucleotides in DNA provides a code for constructing a protein.
Transcription produces genetic messages in the form of RNA
Overview of transcription
– An RNA molecule is transcribed from a DNA template by a process that resembles the synthesis of a DNA strand during DNA replication.
– RNA nucleotides are linked by the transcription enzyme RNA polymerase.
– Specific sequences of nucleotides along the DNA mark where transcription begins and ends.
– The “start transcribing” signal is a nucleotide sequence called a promoter.
Transcription produces genetic messages in the form of RNA
– Transcription begins with initiation, as the RNA polymerase attaches to the promoter.
– During the second phase, elongation, the RNA grows longer.
– As the RNA peels away, the DNA strands rejoin.
– Finally, in the third phase, termination, the RNA polymerase reaches a sequence of bases in the DNA template called a terminator, which signals the end of the gene.
– The polymerase molecule now detaches from the RNA molecule and the gene.
DNA
Cap
Exon Intron Exon
RNAtranscriptwith capand tail
ExonIntron
TranscriptionAddition of cap and tail
Introns removed Tail
Exons spliced together
Coding sequenceNUCLEUS
CYTOPLASM
mRNA
POST-TRANSCRIPTIONAL MODIFICATION
Eukaryotic mRNA
– RNA splicing
– Additions- cap and tail
Post-Transcriptional Modification:Eukaryotic RNA is processed
before leaving the nucleus as mRNA
Post-Transcriptional Modification:Eukaryotic RNA is processed
before leaving the nucleus as mRNA
Messenger RNA (mRNA)
– encodes amino acid sequences and
– conveys genetic messages from DNA to the translation machinery of the cell, which in
– prokaryotes, occurs in the same place that mRNA is made, but in
– eukaryotes, mRNA must exit the nucleus via nuclear pores to enter the cytoplasm.
– Eukaryotic mRNA has
– introns, interrupting sequences that separate
– exons, the coding regions.
Genetic information written in codons is translated into amino acid sequences
The flow of information from gene to protein is based on a triplet code: the genetic instructions for the amino acid sequence of a polypeptide chain are written in DNA and RNA as a series of nonoverlapping three-base “words” called codons.
Each amino acid is specified by a codon.– 64 codons are possible.
– Some amino acids have more than one possible codon.
Transfer RNA molecules serve as interpreters during translation
Transfer RNA (tRNA)
Ribosomes build polypeptides
rRNA and proteins make up the ribosome.
Translation occurs on the surface of the ribosome
The genetic code dictates how codons are translated into amino acids
Characteristics of the genetic code
– _______ nucleotides specify one amino acid.
– 61 codons correspond to amino acids.
– AUG is the start codon; codes for methionine and signals the start of transcription.
– 3 “stop” codons signal the end of translation; _____, ____, ____
– __________- with more than one codon for some amino acids
– _______________- the genetic code is shared by organisms from the simplest bacteria to the most complex plants and animals
GENETIC CODE
Practice
• DNA is TACAGGCGATGGATT
• mRNA is ____________________
• Divide into codons (reading frames)
• Amino acids coded for are:
An initiation codon marks the start of an mRNA message (Initiation)
Translation can be divided into the same three phases as transcription:
1. initiation,
2. elongation, and
3. termination.
Initiation brings together…
An initiation codon marks the start of an mRNA message (Initiation)
Initiation establishes where translation will begin.
Initiation occurs in two steps.
1. An mRNA molecule binds to a small ribosomal subunit and the first tRNA binds to mRNA at the start codon.
– The start codon reads AUG and codes for methionine.
– The first tRNA has the anticodon UAC.
2. A large ribosomal subunit joins the small subunit, allowing the ribosome to function.
– The first tRNA occupies the P site, which will hold the growing peptide chain.
– The A site is available to receive the next tRNA.
Elongation adds amino acids to the polypeptide chain
Once initiation is complete, amino acids are added one by one to the first amino acid.
Elongation is the addition of amino acids to the polypeptide chain.
Each cycle of elongation has three steps.
1. Codon recognition: The anticodon of an incoming tRNA molecule, carrying its amino acid, pairs with the mRNA codon in the A site of the ribosome.
2. Peptide bond formation: The new amino acid is joined to the chain.
3. Translocation: tRNA is released from the P site and the ribosome moves tRNA from the A site into the P site.
Elongation adds amino acids to the polypeptide chain
Polypeptide
mRNA
Codon recognition
Anticodon
Aminoacid
Codons
Psite
Asite
1
Peptide bond2
formation
Translocation3
Newpeptidebond
Stopcodon
mRNAmovement
Elongation
Termination stage of translation, when– the ribosome reaches a stop codon,
– the completed polypeptide is freed from the last tRNA, and
– the ribosome splits back into its separate subunits.
Elongation adds amino acids to the polypeptide chain
until a stop codon; Termination
Other Helpful Animations….
http://highered.mcgraw-hill.com/sites (there are selections at this site that will help with replication, transcription and translation)
DNATranscription
mRNARNApolymerase
Transcription
Translation
Amino acid
Enzyme
CYTOPLASM
Amino acidattachment
2
1
3
4
tRNA
ATP
Anticodon
Initiation ofpolypeptide synthesis
Elongation
Largeribosomalsubunit
InitiatortRNA
Start Codon
mRNA
Growingpolypeptide
Smallribosomalsubunit
New peptidebond forming
Codons
mRNA
Polypeptide
Termination5
Stop codon
Review: The flow of Geneticinformation in the cell is
DNA
RNA
Protein
Mutations can change the meaning of genes
A mutation is any change in the nucleotide sequence of DNA.
Mutations can involve
– large chromosomal regions or
– just a single nucleotide pair.
Mutations can be spontaneous (mistakes during replication) or caused by mutagens.
Examples- UV light, chemicals
. A mutation can be:– Harmful, giving rise to cancers.– Cause genetic disorders (if in sperm or egg)
– Create new traits/variation
in the species
Mutations can change the meaning of genes
Normal hemoglobin DNA Mutant hemoglobin DNA
mRNA mRNA
Sickle-cell hemoglobinNormal hemoglobin
Glu Val
C T T
G A A
C T
G A
A
U
Normalgene
Nucleotidesubstitution
Nucleotidedeletion
Nucleotideinsertion
Inserted
Deleted
mRNAProtein Met
Met
Lys Phe
Lys Phe
Ala
Ala
Gly
Ser
A U G A A G U U U G G C G C A
G C G C AAG U U UA U G A A
Met Lys Ala HisLeu
G U UA U G A A G G C G C A U
U
Met Lys Ala HisLeu
G U UA U G A A G G CU G G C
1. Compare the structures of DNA and RNA.
2. State the contributions of Chargaff, Franklin, Wilkins, Watson and Crick to our understanding of DNA.
3. Describe the process of DNA replication. State the role of helicase, DNA polymerases, primase, and DNA ligase
4. Describe the general purpose of protein synthesis; relate DNA sequence to the specific protein produced.
You should now be able to
5. State the general flow of genetic information as genes are expressed.
6. Explain transcription and how mRNA is produced using DNA.
7. Explain how eukaryotic RNA is processed before leaving the nucleus.
8. Discuss the role of mRNA, tRNA and rRNA in translation.
9. Explain translation; initiation, elongation, translocation and termination.
You should now be able to
10. Describe the structure and function of ribosomes
11. .Define mutation, causes of mutations, and potential consequences.
12. State the amino acid sequence in a polypeptide given the mRNA.
You should now be able to
© 2012 Pearson Education, Inc.
DNA
(b)
is a polymermade from
monomers called
is performedby an
enzyme called(c)
(a)
(d)
(e)
(f)
comesin three
kinds called
use amino-acid-bearingmolecules called
is performedby structures
called (h)
molecules arecomponents of
RNA
Protein
(g)
(i)one or more polymers
made frommonomers called