ESSENTIAL KNOWLEDGE 3.A.1: DNA, and in some cases RNA, is the primary source of heritable information.
Feb 23, 2016
ESSENTIAL KNOWLEDGE 3.A.1:
DNA, and in some cases RNA, is the primary source of heritable information.
iv. tRNA brings the correct amino acid to the correct place on the mRNA.
v. The amino acid is transferred to the growing peptide chain.
vi. The process continues along the mRNA until a “stop” codon is reached.
vii. The process terminates by release of the newly synthesized peptide/protein.
d. Phenotypes are determined through protein activities. To foster student understanding of this concept, instructors
can choose an illustrative example such as: • Enzymatic reactions • Transport by proteins • Synthesis • Degradation e.
GENETIC INFORMATION IS TRANSMITTED FROM ONE GENERATION TO THE NEXT THROUGH DNA OR RNA.
Genetic information is stored in and passed to subsequent generations through DNA molecules and, in some cases, RNA molecules.
RNA WORLD HYPOTHESIS The RNA world
hypothesis proposes that self-replicating ribonucleic acid (RNA) molecules were precursors to current life.
RNA stores genetic information like DNA, and catalyzes chemical reactions like an enzyme protein.
Many viruses also store and transmit RNA
CHROMOSOMES Noneukaryotic organisms have circular
chromosomes, while eukaryotic organisms have multiple linear chromosomes, although in biology there are exceptions to this rule.
linear circular
PROKARYOTES, VIRUSES AND EUKARYOTES CAN CONTAIN PLASMIDS, WHICH ARE SMALL EXTRA-CHROMOSOMAL, DOUBLE-STRANDED CIRCULAR DNA MOLECULES.
THE PROOF THAT DNA IS THE CARRIER OF GENETIC INFORMATION INVOLVED A NUMBER OF IMPORTANT HISTORICAL EXPERIMENTS. THESE INCLUDE:i. Contributions of Watson, Crick, Wilkins, and
Franklin on the structure of DNAii. Avery-MacLeod-McCarty experimentsiii. Hershey-Chase experiment
THE GREAT DEBATEWhich chemical is used to store and transmit
genetic information?
Protein or DNA
Most Scientists of the day(early and mid 1900(s) agreed that the substance must be
protein.
EVIDENCE FOR DNA AS GENETIC MATERIAL Griffith, 1928 - In his work with
Streptococcus pneumoniae, Griffith realized that some “transforming” agent was exchanged between bacteria which enabled to acquire traits from one another.
The use of heat to inactivate cells suggested that the agent was not protein.
This phenomenon is now called transformation - a change in phenotype by taking genetic material from the environment.
GRIFFITH EXPERIMENT
AVERY-MACLEOD-MCCARTY EXPERIMENTS Took Griffiths experiment a step further by
isolating different chemical to see which one would transform bacteria.
Avery, et al., 1944 - isolated various chemicals from bacteria and used them to try transform bacteria. Only DNA worked.
VIRUSES ARE MADE OF NUCLEIC ACID AND PROTEIN
HERSHEY CHASE EXPERIMENT (1952)
CHARGAFF (1947)
Adenine pairs Thymine; Cytosine pairs Guanine
If a mixture made from cells contained 20% Adenine, then what is the percentage of Guanine?
STRUCTURE OF DNA Wilkins and
Franklin used X-ray diffraction to attempt to find the structure of DNA.
THE STRUCTURE OF DNA WAS DISCOVERED
Watson and Crick (1953)
Double Helix Sides: phosphate
and sugar Rungs:
nitrogenous bases held together by hydrogen bonds
DNA NUCLEOTIDE – MONOMER OF DNA AND RNA
OO=P-O O
Phosphate Group
NNitrogenous base (A, G, C, or T)
CH2
O
C1C4
C3 C2
5
Sugar(deoxyribose)
NITROGENOUS BASES
Double ring PURINESAdenine (A)Guanine (G)
Single ring PYRIMIDINESThymine (T)Cytosine (C)
T or C
A or G
DNA STRANDS ARE ANTI-PARALLEL
P
P
P
O
O
O
1
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4
5
5
3
3
5
P
P
PO
O
O
1
2 3
4
5
5
3
5
3
G C
T A
STRUCTURE OF DNA
DNA REPLICATION ENSURES CONTINUITY OF HEREDITARY INFORMATION. Replication is a semiconservative process;
that is, one strand serves as the template for a new, complementary strand.
DNA REPLICATION (SEMICONSERVATIVE MODEL)
ORIGIN OF REPLICATION Origin of replication (“bubbles”): beginning of
replication Replication fork: ‘Y’-shaped region where new strands
of DNA are elongating Helicase:catalyzes the untwisting of the DNA at the
replication fork DNA polymerase:catalyzes the elongation of new DNA
DNA REPLICATION
Antiparallel nature: • sugar/phosphate backbone runs in opposite directions (Crick); • one strand runs 5’ to 3’, while the other runs 3’ to 5’; • DNA polymerase only adds nucleotides at the free 3’ end, forming new DNA strands in the 5’ to 3’ direction only
DNA REPLICATION Leading strand:
synthesis toward the replication fork (only in a 5’ to 3’ direction from the 3’ to 5’ master strand)
Lagging strand: synthesis away from the replication fork (Okazaki fragments); joined by DNA ligase (must wait for 3’ end to open; again in a 5’ to 3’ direction)
Initiation: Primer (short RNA sequence~w/primase enzyme), begins the replication process
SIMILAR TO ATP!! The energy to add new nucleotides comes from the substrates themselves which are nucleoside triphosphates.
The loss of two phosphates from the substrate provides the energy to drive the reaction.
RETROVIRUSES REVERSE THE NORMAL FLOW OF GENETIC INFORMATION Genetic information in retroviruses is a
special case and has an alternate flow of information: from RNA to DNA, made possible by reverse transcriptase, an enzyme that copies the viral RNA genome into DNA.
This DNA integrates into the host genome and becomes transcribed and translated for the assembly of new viral progeny.
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RNA
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RNA DIFFERS FROM DNA1. RNA has a sugar ribose
DNA has a sugar deoxyribose2. RNA contains the base uracil (U)
DNA has thymine (T)3. RNA molecule is single-stranded
DNA is double-stranded
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STRUCTURE OF RNA
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. THREE TYPES OF RNA
Messenger RNA (mRNA) carries genetic information to the ribosomes
Ribosomal RNA (rRNA), along with protein, makes up the ribosomes
Transfer RNA (tRNA) transfers amino acids to the ribosomes where proteins are synthesized
THE 4TH TYPE OF RNA! THE ROLE OF RNAI INCLUDES REGULATION OF GENE EXPRESSION AT THE LEVEL OF MRNA TRANSCRIPTION.
RNA interference (RNAi) is a biological process in which RNA molecules inhibit gen expression, typically by causing the destruction of specific mRNA molecules.
Protein Synthesis
CENTRAL DOGMA Genetic information flows from a sequence of
nucleotides in a gene to a sequence of amino acids in a protein.
THE TRIPLET CODE
The genetic instructions for a polypeptide chain are ‘written’ in the DNA as a series of 3-nucleotide ‘words’
Codons ‘U’ (uracil) replaces
‘T’ in RNA
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NAME THE AMINO ACIDS
GGG?UCA?CAU?GCA?AAA?
TRANSCRIPTION The enzyme RNA-polymerase reads the DNA
molecule in the 3' to 5' direction and synthesizes complementary mRNA molecules that determine the order of amino acids in the polypeptide.
TRANSCRIPTION RNA polymerase: pries
DNA apart and hooks RNA nucleotides together from the DNA code
Promoter region on DNA: where RNA polymerase attaches and where initiation of RNA begins
Terminator region: sequence that signals the end of transcription
Transcription unit: stretch of DNA transcribed into an RNA molecule
TRANSCRIPTION Initiation~ transcription
factors mediate the binding of RNA polymerase to an initiation sequence (TATA box)
Elongation~ RNA polymerase continues unwinding DNA and adding nucleotides to the 3’ end
Termination~ RNA polymerase reaches terminator sequence
MRNA MODIFICATIONS In eukaryotic cells the mRNA transcript
undergoes a series of enzyme-regulated modifications.
• Addition of a poly-A tail• Addition of a GTP cap• Excision of introns
MRNA MODIFICATION 1) 5’ cap: modified guanine; protection; recognition site for
ribosomes 2) 3’ tail: poly(A) tail (adenine); protection; recognition;
transport 3) RNA splicing: exons (expressed sequences) kept,introns
(intervening sequences) spliced out; spliceosome
TRANSLATION Translation of the mRNA occurs in the
cytoplasm on the ribosome. In prokaryotic organisms, transcription is
coupled to translation of the message. Translation involves energy and many steps, including initiation, elongation and termination.
TRANSLATION mRNA from
nucleus is ‘read’ along its codons by tRNA’s anticodons at the ribosome
tRNA anticodon (nucleotide triplet); amino acid
TRANSLATION rRNA
site of mRNA codon & tRNA anticodon coupling
P site holds the tRNA carrying the growing polypeptide chain
A site holds the tRNA carrying the next amino acid to be added to the chain
E site discharged tRNA’s
TRANSLATION Initiation~
union of mRNA, tRNA, small ribosomal subunit; followed by large subunit
Elongation~ •codon recognition •peptide bond formation •translocation
Termination~ ‘stop’ codon reaches ‘A’ site
Polyribosomes: translation of mRNA by many ribosomes (many copies of a polypeptide very quickly)
MUTATIONS: GENETIC CHANGES IN A CELL
Point mutations…. Changes in 1 or a few base
pairs in a single gene Base-pair substitutions: •silent
mutations no effect on protein
•missense ∆ to a different amino acid (different protein)
•nonsense ∆ to a stop codon and a
nonfunctional protein Base-pair insertions or
deletions: additions or losses of nucleotide pairs in a gene; alters the ‘reading frame’ of triplets~frameshift mutation
Mutagens: physical and chemical agents that change DNA
GENETIC ENGINEERING TECHNIQUES CAN MANIPULATE THE HERITABLE INFORMATION OF DNA AND, IN SPECIAL CASES, RNA. • Electrophoresis • Plasmid-based transformation • Restriction enzyme analysis of DNA • Polymerase Chain Reaction (PCR)
RESTRICTION ENZYMES ANALYSIS A restriction enzyme (or restriction
endonuclease) is an enzyme that cuts DNA at or near specific recognition nucleotide sequences known as restriction sites.
Fragments are separated using gel electrophoresis
ELECTROPHORESIS The separation of DNA fragments according
to charge and size. Smaller fragments migrate through the jell at
a faster rate than larger fragments. The gels are stained an viewed under UV
light.
PCR The polymerase chain reaction (PCR) is a
biochemical technology in molecular biology to amplify a single or a few copies of a piece of DNA across several orders of magnitude, generating thousands to millions of copies of a particular DNA sequence.
Developed in 1983 by Kary Mullis, PCR is now a common and often indispensable technique used in medical and biological research labs for a variety of applications
GENETICALLY ENGINEERED PRODUCTS: Genetically modified foods Transgenic animals Cloned animals Pharmaceuticals, such as human insulin or
factor X