Download Chapter 24 PPT

24.1 DNA Structure and Replication
• Hershey-Chase Experiments (1952)
– Demonstrated that DNA is the genetic material
– DNA stands for Deoxyribonucleic Acid
Hershey-Chase Experiments
24.1 DNA Structure and Replication
• Structure of DNA
– James Watson and Francis Crick determined the
structure of DNA in 1953
– DNA is a chain of nucleotides
– Each nucleotide is a complex of three subunits
• Phosphoric acid (phosphate)
• A pentose sugar (deoxyribose)
• A nitrogen-containing base
24.1 DNA Structure and Replication
• Structure of DNA
– Four Possible Bases
• Adenine (A) - a purine
• Guanine (G) - a purine
• Thymine (T) - a pyrimidine
• Cytosine (C) - a pyrimidine
– Complimentary Base Pairing
• Adenine (A) always pairs with Thymine (T)
• Guanine (G) always pairs with Cytosine (C)
Overview of DNA Structure
24.1 DNA Structure and Replication
• Replication of DNA
– Semi-conservative replication
• Each daughter DNA molecule consists of one new chain of
nucleotides and one from the parent DNA molecule
– The two daughter DNA molecules will be identical to
the parent molecule
24.1 DNA Structure and Replication
• Replication of DNA
– Before replication begins, the two strands of the parent molecule
are hydrogen-bonded together
– Enzyme DNA helicase unwinds and “unzips” the doublestranded DNA
– New complementary DNA nucleotides fit into place along divided
strands by complementary base pairing. These are positioned
and joined by DNA polymerase
– DNA ligase repairs any breaks in the sugar-phosphate backbone
– The Two double helix molecules identical to each other and to
the original DNA molecule
Overview of DNA Replication
Ladder Configuration and DNA
Replication
24.2 Gene Expression
• Gene:
A segment of DNA that specifies the
amino acid sequence of a polypeptide
• DNA does not directly control protein synthesis,
instead its information is transcribed into RNA
24.2 Gene Expression
• RNA (ribonucleic acid)
24.2 Gene Expression
• Three Classes of RNA
– Messenger RNA (mRNA)
• Takes a message from DNA to the ribosomes
– Ribosomal RNA (rRNA)
• Makes up ribosomes (along with proteins)
– Transfer RNA (tRNA)
• Transfers amino acids to ribosomes
24.2 Gene Expression
• Gene Expression Requires Two Steps:
– Transcription
– Translation
24.2 Gene Expression
• Transcription
– During transcription, a segment of the DNA serves as a template
for the production of an RNA molecule
– Messenger RNA (mRNA)
• RNA polymerase binds to a promoter
• DNA helix is opened so complementary base pairing can
occur
• RNA polymerase joins new RNA nucleotides in a sequence
complementary to that on the DNA
24.2 Gene Expression
• Transcription
– Processing of mRNA
• Primary mRNA becomes mature mRNA
• Contains bases complementary to both intron and exon
segments of DNA
– Introns are intragene segments
– Exons are the portion of a gene that is expressed
• Intron sequences are removed, and a poly-A tail is added
– Ribozyme splices exon segments together
Transcription of DNA to form mRNA
mRNA Processing
24.2 Gene Expression
• Translation
– The Genetic Code
• Triplet code- each 3-nucleotide unit of a mRNA molecule is
called a codon
• There are 64 different mRNA codons
– 61 code for particular amino acids
» Redundant code-some amino acids have numerous code
words
» Provides some protection against mutations
– 3 are stop codons signal polypeptide termination
Messenger RNA Codons
24.2 Gene Expression
• Transfer RNA
– tRNA transports amino acids to the ribosomes
– Single stranded nucleic acid that correlates a specific
nucleotide sequence with a specific amino acid
– Amino acid binds to one end, the opposite end has an
anticodon
– the order of mRNA codons determines the order in
which tRNA brings in amino acids
Transfer RNA: Amino Acid Carrier
24.2 Gene Expression
• Ribosome and Ribosomal RNA
– Ribosome has a binding site for mRNA and for 2
tRNAs
– Facilitate complementary base pairing
– Ribosome moves along mRNA and new tRNAs come
in and line up in order
– This brings amino acids in line in a specific order to
form a polypeptide
– Several ribosomes may move along the same mRNA
• Multiple copies of a polypeptide may be made
• The entire complex is called a polyribosome
Polyribosome Structure and Function
Overview of Gene Expression
24.2 Gene Expression
• Translation Requires Three Steps
– Initiation (requires energy)
– Elongation (requires energy)
– Termination
Initiation
Elongation
Termination
Summary of Gene Expression
24.2 Gene Expression
• Genes and Gene Mutations
– A gene mutation is a change in the sequence of
bases within a gene.
– Gene mutations can lead to malfunctioning proteins in
cells.
24.2 Gene Expression
• Genes and Gene Mutations
– Causes of Mutations
• Errors in replication
– Rare
– DNA polymerase “proofreads” new strands and errors are
cleaved out
• Mutagens
– Environmental influences
– Radiation, UV light, chemicals
– Rate is low because DNA repair enzymes monitor and repair
DNA
• Transposons
– “jumping genes”
– Can move to new locations and disrupt sequences
Transposon
24.2 Gene Expression
• Types of Mutations
– Frameshift Mutations
• One or more nucleotides are inserted or deleted
• Results in a polypeptide that codes for the wrong sequence
of amino acids
– Point Mutations
• The substitution of one nucleotide for another
– Silent mutations
– Nonsense mutations
– Missense mutations
24.3 DNA Technology
• The Cloning of a Gene
– Cloning:
Production of many identical copies of
an organism through some asexual means.
– Gene Cloning:
The production of many identical
copies of a single gene
– Two Ways to Clone a Gene:
– Recombinant DNA
– Polymerase Chain Reaction
Cloning of a Human Gene /
Recombinant DNA
24.3 DNA Technology
• Using Recombinant DNA Technology
– Restriction enzymes breaks open a plasmid vector at
specific sequence of bases “sticky ends”
– Foreign DNA that is to be inserted is also cleaved
with same restriction enzyme so ends match
– Foreign DNA is inserted into plasmid DNA and “sticky
ends” pair up
– DNA ligase seals them together
Restriction Enzymes and Stick Ends
24.3 DNA Technology
• Polymerase Chain Reaction
– Amplifies a targeted DNA sequence
– Requires DNA polymerase, a set of primers, and a
supply of nucleotides
• Primers are single stranded DNA sequences that start
replication process
– Amount of DNA doubles with each replication cycle
– Process is now automated
24.3 DNA Technology
• DNA Fingerprinting
– Permits identification of individuals and their relatives
– Based on differences between sequences in
nucleotides between individuals
– Detection of the number of repeating segments
(called repeats) are present at specific locations in
DNA
• Different numbers in different people
• PCR amplifies only particular portions of the DNA
• Procedure is performed at several locations to identify
repeats
DNA Fingerprints
24.3 DNA Technology
• Biotechnology
– Biotechnology uses natural biological systems to
create a product or to achieve a goal desired by
humans.
– Transgenic organisms have a foreign gene inserted
into their DNA
24.3 DNA Technology
• Transgenic Bacteria
– Medical Uses: Production of Insulin, Human Growth
Hormone, Tissue Plasminogen Activator, Hepatitis B
Vaccine
– Agricultural Uses: Bacteria that protects plants from
freezing, bacteria that protect plant roots from insects
– Environmental: Bacteria that degrade oil (clean up
after oil spills), bacteria that remove sulfur from coal
24.3 DNA Technology
• Transgenic Plants
– Plants have been engineered to secrete a toxin that
kills insects
– Plants have been engineered to be resistant to
herbicides
24.3 DNA Technology
• Transgenic Animals
– Fish, cows, pigs, rabbits and sheep have been
engineered to produce human growth hormone in
order to increase size of the animals
Transgenic Animals