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

Hershey-Chase Experiments


• 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


• 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


• 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


• 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

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


• RNA (ribonucleic acid)


• 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


• Gene Expression Requires Two Steps:

– Transcription

– Translation


• 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


• 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


• 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


• 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


• 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


• Translation Requires Three Steps

– Initiation (requires energy)

– Elongation (requires energy)

– Termination

Initiation

Elongation

Termination

Summary of 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.


• 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


• 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

Documents

24.1 DNA Structure and Replication