DNA: The Genetic Material and Structure
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Questions and Answers

Which of the following is NOT a level at which gene expression is regulated in eukaryotes?

  • Translational level
  • Transcriptional level
  • Metabolic level (correct)
  • Processing level
  • The lac operon includes genes responsible for the hydrolysis of sucrose.

    False

    What is the function of the i gene in the lac operon?

    It codes for the repressor of the lac operon.

    In eukaryotic cells, the _____ helps RNA polymerase recognize start sites for transcription.

    <p>promoter</p> Signup and view all the answers

    Match the following lac operon genes with their functions:

    <p>i = Codes for repressor z = Codes for β-galactosidase y = Codes for permease a = Encodes transacetylase</p> Signup and view all the answers

    What role does lactose play in the regulation of the lac operon?

    <p>It is an inducer.</p> Signup and view all the answers

    Eukaryotic gene expression is solely regulated by transcription factors.

    <p>False</p> Signup and view all the answers

    What happens to the lac operon when glucose is present in the growth medium?

    <p>The lac operon remains off.</p> Signup and view all the answers

    Which RNA polymerase is responsible for transcribing mRNA in eukaryotes?

    <p>RNA polymerase II</p> Signup and view all the answers

    In eukaryotes, primary transcripts are functional and ready for protein synthesis.

    <p>False</p> Signup and view all the answers

    What process involves the removal of introns and the joining of exons in eukaryotic cells?

    <p>Splicing</p> Signup and view all the answers

    RNA polymerase III is responsible for transcribing _______.

    <p>tRNA</p> Signup and view all the answers

    How many adenylate residues are typically added at the 3’-end of hnRNA during the tailing process?

    <p>200-300</p> Signup and view all the answers

    Match the following RNA polymerases with their corresponding functions:

    <p>RNA Polymerase I = Transcribes rRNAs RNA Polymerase II = Transcribes mRNA RNA Polymerase III = Transcribes tRNAs RNA Polymerase in organelles = Transcribes mitochondrial genes</p> Signup and view all the answers

    Alternative RNA splicing allows for the creation of multiple mRNA molecules from a single primary transcript.

    <p>True</p> Signup and view all the answers

    What phenomenon is proposed to explain the low number of human genes compared to the high complexity of human proteins?

    <p>Alternative RNA splicing</p> Signup and view all the answers

    Which statement accurately describes the genetic code?

    <p>The codon is read in a contiguous manner without punctuation.</p> Signup and view all the answers

    Point mutations only involve changes in multiple base pairs.

    <p>False</p> Signup and view all the answers

    What are the three codons that function as stop codons?

    <p>UAA, UAG, UGA</p> Signup and view all the answers

    Francis Crick postulated the presence of an ___ that reads the genetic code and binds to amino acids.

    <p>adapter molecule</p> Signup and view all the answers

    Match the following terms with their definitions:

    <p>Point mutation = Change of a single base pair in the gene Frameshift mutation = Insertion or deletion that alters the reading frame tRNA = Adapter molecule for translating codons to amino acids Codon = A triplet of nucleotides that codes for an amino acid</p> Signup and view all the answers

    Which of the following statements about tRNA is correct?

    <p>tRNA is shaped like a clover leaf.</p> Signup and view all the answers

    The genetic code is highly specific and differs across organisms.

    <p>False</p> Signup and view all the answers

    What is the dual function of AUG in the genetic code?

    <p>It codes for Methionine and acts as the initiator codon.</p> Signup and view all the answers

    Study Notes

    DNA: The Genetic Material

    • DNA is the genetic material
    • Bacteriophage φ×174 has 5386 nucleotides
    • Bacteriophage λ has 48,502 base pairs (bp)
    • Escherichia coli has 4.6 × 109 bp
    • Human haploid DNA has 3.3 × 109 bp

    DNA Structure

    • Polynucleotide chain consists of three components:
      • Nitrogenous base
      • Pentose sugar (ribose in RNA, deoxyribose in DNA)
      • Phosphate group
    • Nitrogenous bases are of two types:
      • Purines (Adenine, Guanine)
      • Pyrimidines (Cytosine, Thymine in DNA, Uracil in RNA)
    • N-glycosidic linkage connects a nitrogenous base to the 1' carbon of the pentose sugar, forming a nucleoside
    • Examples of Nucleosides: Adenosine, Deoxyadenosine, Guanosine, Deoxyguanosine, Cytidine, Deoxycytidine, Uridine, Deoxythymidine
    • Phosphoester linkage connects the phosphate group to the 5' carbon of the sugar, forming a nucleotide
    • 3'-5' phosphodiester linkage connects nucleotides to create a polynucleotide chain
    • Thymine is a 5-methyl uracil

    DNA Structure (page 2)

    • 3' end of a polynucleotide chain has a free hydroxyl group (-OH) attached to the 3' carbon of the deoxyribose sugar
    • In RNA, the 2' carbon of the ribose sugar also has a hydroxyl group
    • DNA has a sugar-phosphate backbone
    • Nitrogenous bases project from the backbone in the 1 and 3 positions, and are perpendicular to the backbone

    Chargaff's Rules

    • Adenine = Thymine (A=T)
    • Guanine = Cytosine (G=C)
    • Adenine + Guanine = Thymine + Cytosine (A+G = T+C)

    Double Helix Model of DNA Structure

    • DNA consists of two polynucleotide chains wound around each other to form a double helix
    • Sugar-phosphate backbones are on the outside
    • Nitrogenous bases are on the inside, paired through hydrogen bonds
    • Base pairing: A with T, and G with C
    • Pitch of helix: 3.4 nm
    • Distance between base pairs: 0.34 nm
    • Antiparallel polarity: Strands run in opposite directions
    • Coiled in a right-handed fashion

    Packaging of DNA Helix

    • DNA is not scattered; it's held within a region termed the nucleoid in prokaryotes
    • In eukaryotes, DNA is packaged with proteins called histones
    • Histones have a positive charge that attracts the negatively charged DNA
    • A nucleosome consists of DNA wound around an octamer of eight histone proteins
    • Nucleosomes form a string of beads ("beads-on-a-string") that is further compacted into chromatin fibers
    • Euchromatin: less dense; transcriptionally active
    • Heterochromatin: more dense; transcriptionally inactive

    The Search for Genetic Material

    • 1928: Frederick Griffith identified a "transforming principle" in Streptococcus pneumoniae that could transform non-pathogenic bacteria into pathogenic ones
    • 1952: Alfred Hershey and Martha Chase demonstrated that DNA is the genetic material of viruses

    Properties of Genetic Material: DNA versus RNA

    • DNA is more stable than RNA due to the presence of thymine instead of uracil at one of the bases
    • RNA is more catalytic than DNA
    • RNA was the first genetic material in essential life processes

    History of DNA (page 5)

    • DNA identified as an acidic substance in the nucleus in 1869 by Friedrich Miescher

    The Hershey-Chase Experiment (page 6)

    • Confirmed that DNA is the genetic material

    Models of DNA Replication (page 8)

    • Conservative model: Parental strands stay together; new strands bind together
    • Semi-conservative model: Parental strands separate; each serves as a template to form a new strand
    • Dispersive model: Pieces of old and new DNA are mixed in both new strands

    The Machinery for DNA Replication (page 9)

    • DNA replication is a costly process, utilizing deoxyribonucleotide phosphates
    • DNA dependent DNA polymerase catalyzes the polymerization of deoxyribonucleotides using a DNA template
    • DNA polymerase moves along a template strand and polymerizes in a 5' to 3' direction
    • Synthesis of one strand is continuous; synthesis of other strand is discontinuous, creating Okazaki fragments
    • DNA ligase joins the Okazaki fragments

    Prokaryotic DNA Replication (page 10)

    Proteins initiate replication, the strands separate, and replication proceeds in both directions until the entire molecule is copied creating replication 'bubbles'

    Eukaryotic DNA Replication (page 10)

    • Multiple replication origins lead to multiple replication bubbles that fuse together to speed up the replication process

    Transcription (page 12)

    • Transcription is the process of making RNA from a DNA template
    • Involves RNA polymerase binding to a promoter, unwinding the DNA double helix, and synthesizing a complementary RNA sequence (follows base-pairing rules but replaces T with U)
    • Elongation: RNA polymerase moves along DNA
    • Termination: RNA polymerase reaches a termination sequence

    Transcription Unit (page 12)

    • Promoter: RNA polymerase binding site
    • Structural gene: codes for a protein
    • Terminator: Transcription termination signal

    Eukaryotic Transcription (page 13)

    • Eukaryotic cells have three types of RNA polymerases (I, II, and III)
    • Transcription factors are needed along with RNA polymerase in eukaryotes

    The Transcription Process (page 14)

    • Initiation: RNA polymerase binds to the promoter
    • Elongation: RNA polymerase moves along DNA, synthesizing RNA
    • Termination: RNA polymerase reaches a termination sequence

    RNA Processing (page 16)

    • Primary transcripts (pre-mRNA) contain both exons and introns and are non-functional.
    • Splicing: Removal of introns, joining exons to form mature mRNA.
    • Capping: Addition of a modified nucleotide to the 5' end of pre-mRNA, making it stable
    • Tailing: Addition of a string of adenine nucleotides to the 3' end of pre-mRNA, making it stable

    Alternative RNA Splicing (page 16)

    • Multiple mature mRNA molecules can be produced from a single pre-mRNA
    • Different combinations of exons are created during splicing, increasing the number of proteins that a single gene can produce

    Genetic Code (page 17)

    • Three-base codons specify amino acids
    • Codons code for specific amino acids
    • Some codons act as stop signals, signaling the end of translation

    tRNA—The Adapter Molecule (page 18)

    • tRNA acts as translator to transfer amino acids from cytoplasm to ribosomes during translation
    • tRNA has an anticodon loop that complements the codon on mRNA

    Translation (page 19)

    • Translation: the process of synthesizing a polypeptide using mRNA as a template in the ribosomes
    • Initiation, elongation, and termination
    • Amino acids are linked together to form a polypeptide chain following the instructions in mRNA
    • Ribosomes catalyze the formation of peptide bonds
    • Elongation: tRNA molecules deliver the appropriate amino acids and the ribosome adds them to the growing polypeptide chain until a termination codon is reached
    • Termination: the process concludes once a stop codon is encountered

    Regulation of Gene Expression (page 20)

    • Genes are regulated, adjusting expression in response to internal and external factors to control production of proteins
    • The availability of resources, cellular conditions, and external factors influence expression
    • Regulation of genes varies with prokaryotes and eukaryotes

    The Lac Operon (page 20)

    • Lac operon is a set of genes involved in lactose utilization in bacteria.
    • It consists of a regulatory gene (i gene) and three structural genes (z, y, and a genes)
    • The i gene codes for the lac repressor protein
    • The z gene codes for β-galactosidase, an enzyme that breaks down lactose
    • The y gene codes for permease, a protein that transports lactose into the cell
    • The a gene codes for transacetylase, involved in lactose metabolism
    • Operon is turned off when lactose is absent, repressor binds to the operator preventing RNA polymerase from transcribing downstream genes; active repressor blocks RNA polymerase binding
    • Operon is turned on when lactose is present, lactose binds to the repressor, inactivating it
    • The repressor can't bind to the operator; RNA polymerase can transcribe the structural, Z, Y, A, genes; the protein products, which function in lactose utilization, are made

    Human Genome Project (HGP) (page 22)

    • Launched in 1990, aimed at mapping and sequencing the entire human genome
    • 3 billion base pairs in the human genome
    • Estimated to have 25,000 genes

    Expressed Sequence Tags (ESTs) (page 22)

    • ESTs are sequences of expressed genes, which are assigned to different functional categories

    DNA Fingerprinting (page 23)

    • Repetitive DNA regions are used to identify individuals
    • Techniques include Southern blot hybridization using radio-labeled VNTR probes (Variable Number of Tandem Repeats)

    CRISPR-Cas9 System (page 23)

    • Gene editing technology
    • Cas9 enzyme cuts DNA at specific sites
    • Guide RNA directs the Cas9 enzyme to the target gene

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    Description

    Explore the essential concepts of DNA as the genetic material and delve into its intricate structure. Understand the components of a polynucleotide chain and the different types of nitrogenous bases involved. This quiz covers key facts about various organisms' DNA and the structural linkages that form nucleotides.

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