Ch. 12.2: Replication of DNA Section objective: Summarize DNA replication.
24.1 DNA Structure and Replication
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Transcript of 24.1 DNA Structure and Replication
24.1 DNA Structure and Replication
• Hershey-Chase Experiments (1952)
– Demonstrated that DNA is the genetic material
– DNA stands for Deoxyribonucleic Acid
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)
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 double-
stranded 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
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
• 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
• 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
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
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
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
24.2 Gene Expression
• Translation Requires Three Steps
– Initiation (requires energy)
– Elongation (requires energy)
– Termination
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
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
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
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
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