CHAPTER 10 Flashcards
STRUCTURE OF GENETIC MATERIAL
- DNA = genetic material
- Transforming factor, 1928, Fredrick Griffith
- Hershey-Chase experiments, 1952, determined heredity material was DNA not protein
- *Studied the simple bacteriophage T2
- *Showed that the virus injects its DNA into host cells and reprograms them to produce more viruses
DNA & RNA
*DNA & RNA = polymers of nucleotides
*Nucleic acids = polynucleotides made of long chains of nucleotide monomers
*Nitrogenous Bases:
1 Single-ring pyramids: T thymine, C cytosine
2 Double-ring purines: A adenine, G guanine
*Sugar-phosphate = backbone
DNA & RNA
*DNA & RNA IDENTICAL EXCEPT FOR 2 THINGS 1 Nitrogenous Bases: **DNA: A, G, C, T **RNA: A, G, C, U 2 Sugars: **DNA: Deoxyribose **RNA: Ribose
DNA
- DNA = Double-stranded helix
- James Watson and Francis Crick worked out the 3-dimensional structure of DNA, based on X-ray crystallography by Rosalind Franklin
- DNA consists of 2 polynucleotide strands wrapped around each other in a double helix
DNA
- In DNA, each base pairs with a complementary partner
- A with T (apple tree)
- G with C (chewing gum)
- Hydrogen bonds between the bases hold the strands together
- Watson-Crick model of DNA suggested a molecular explanation for genetic inheritance
DNA REPLICATION
- DNA Replication depends on a specific pairing
- Watson-Crick model of DNA structure suggested a mechanism for its replication
- DNA strands separate
- Enzymes use each strand as a template to assemble nucleotides into complementary strands
- Each new double helix consists of one old and one new strand
DNA REPLICATION
- DNA Replication begins at specific sites (origins of replication) on the double helix
- Proteins attach and separate the strands
- Replication proceeds in both directions, creating replication bubbles
- *Parent strands open, daughter strands elongate
- Replication occurs simultaneously at many sites
DNA
- DNA’s sugar-phosphate backbones are oriented in opposite directions
- The enzyme DNA polymerase adds nucleotides at only the 3 ends
- *One daughter strand is synthesized as a continuous piece
- *The other strand is synthesized as a series of short pieces
- *The 2 strands are connected by the enzyme DNA ligase
DNA TO RNA TO PROTEIN
- The DNA genotype is expressed as proteins which provide the molecular basis for phenotypic traits
- *The info constituting an organism’s genotype is carried in its sequence of DNA bases
- *A particular gene - a linear sequence of many nucleotides - specifies a particular polypeptide
GENETIC INFO
*Flow of Genetic Info
1 Transcription of the genetic info in DNA into RNA
2 Translation of RNA into polypeptide
*Beadle-Tatum one gene - one enzyme hypothesis
1 Studies of inherited metabolic disorders in mold suggested that phenotype is expressed through proteins
2 A gene dictates productions of a specific enzyme
3 The hypothesis has been restated to one gene - one polypeptide
GENETIC INFO
- Genetic info written in codons, is translated into amino acid sequences
- Genetic info flows from DNA to RNA to Protein
- Nucleotide monomers represent letters in an alphabet that can form words in a language
- *Triplet Code:
- **3-letter word (codons)
- **Each word codes for one amino acid in a polypeptide
GENETIC CODE
- The genetic code specifies the correspondence between RNA codons and amino acids in proteins
- *Includes start and stop codons
- *Redundant but not ambiguous
- Nearly all organisms use exactly the same genetic code
TRANSCRIPTION
- Transcription produces genetic messages in the form of RNA
- *One DNA strand serves as a template for the new RNA strand
- *RNA polymerase constructs the RNA strand in a multistep process
RNA
*RNA strand constructed in a multistep process
*Initiation:
1 RNA polymerase attaches to the promoter
2 Synthesis starts
*Elongation:
1 RNA synthesis continues
2 RNA strand peels away from DNA template
3 DNA strands come back together in transcribed region
*Termination:
1 RNA polymerase reaches a terminator sequence at the end of the gene
2 Polymerase detaches
RNA
- Eukaryotic RNA is processed before leaving the nucleus
- RNA that encodes an amino acid sequence is messenger RNA (mRNA)
- In prokaryotes, transcription and translation both occur in the cytoplasm
- In eukaryotes, RNA transcribed in the nucleus is processed before moving to the cytoplasm for translation
RNA SPLICING
- Non-coding segments called introns are cut out
- Remaining axons are joined to form a continuous coding sequence
- A cap and a tail are added to the ends
RNA
- Transfer RNA molecules serve as interpreters during translation
- Transfer RNA (tRNA) molecules match the right amino acid to the correct codon
- tRNA = a twisted and folded single strand of RNA
- *Anticodon loop at one end recognizes a particular mRNA codon by base pairing
- *Amino acid attachment site is at the other end
- Each amino acid is joined to the correct tRNA by a specific enzyme
RIBOSOMES
- Ribosomes build polypeptides
- A ribosome consists of 2 subunits
- *Each is made up of proteins and ribosomal RNA (rRNA)
- The subunits of a ribosome
- *Hold the tRNA and mRNA close together in binding sites during translation
- *Allow amino acids to be connected into a polypeptide chain
INITIATION CODON
- An initiation code marks the start of an mRNA message
- Initiation phase of translation
- *Brings together mRNA, a specific tRNA, and the 2 subunits of a ribosome
- *Establishes exactly where translation will begin
- *Ensures that mRNA codes are translated in the correct sequence
INITIATION
- Initiation is a 2 step process
- Step 1:
- *mRNA binds to a small ribosomal subunit
- *Initiator tRNA, carrying the amino acid Met, binds to the start codon
- Step 2:
- *A large ribosomal subunit binds to the small one, forming a functional ribosome
- *Initiator tRNA fits into one binding site, the other is vacant for the next tRNA
ELONGATION
*Elongation adds amino acids to the polypeptide chain until a stop codon terminates translation
*Once initiation is complete, amino acids are added one by one in a 3-step elongation process
1 Codon Recognition
2 Peptide Bond Formation
3 Translocation
*Elongation continues until a stop codon reaches the ribosome’s A site, terminating translation
REVIEW
*The flow of genetic info in the cell is DNA -> RNA -> Protein
*Sequence of codons in DNA via the sequence of codons in RNA, spells out the primary structure of a polypeptide
1 Transcription of mRNA from a DNA template
2 Attachment of amino acid to tRNA
3 Initiation of polypeptide synthesis
4 Elongation
5 Termination
MUTATIONS
- Mutation can change the meaning of genes
- Mutation = any change in the nucleotide sequence of DNA
- *Caused by errors in DNA replication or recombination, or by mutagens
- *Can involve large regions of a chromosome or a single base pair
- *Can cause many genetic disease, such as sickle-cell disease
GENETIC MUTATIONS
- 2 General categories of genetic mutations
- Base substitutions replace one base with another
- *Most are harmful but many occasionally have no effect or be beneficial
- Base insertions or deletions alter the reading frame
- *Result is most likely a non-functioning polypeptide
- Mutagenesis caused by spontaneous error or a physical or chemical mutagen
TYPES OF MUTATIONS (SILENT)
- Silent Mutation
- *If mutation causes an mRNA codon to change from GAA to GAG, there would be no effect
- *Both GAA and GAG code for same amino acid
TYPES OF MUTATIONS (MISSENSE)
- Missense Mutation
- *If mutation changes one amino acid to another
- *If changes from GGC (gly) to AGC (ser), it would change from gly (GGC) to ser (AGC)
TYPES OF MUTATIONS (NONSENSE)
- Nonsense Mutation
- *If mutation changes an amino acid codon into a stop codon
- *If changes from AGA (arg) to UGA (stop), it would prematurely terminate the protein
MICROBIAL GENETICS
- Viral DNA may become part of the host chromosome
- *Viruses are infectious particles consisting of nucleic acid enclosed in a protein capsid
- *Virus depend on their host cells for the replication, transcription, and translation of their nucleic acid
- DNA enters host bacterium, circularizes, and enters one of two pathways
LYTIC CYCLE
- Host provides more viruses
* Host cell lyses (breaks opes) to release new viruses
LYSOGENIC CYCLE
- Phage DNA inserted by recombination into the host chromosome, it is now a prophage
- Prophages replicated each time host cell divides, passed on to generations of daughter cells
- Doesn’t destroy host
- Environmental signal may trigger switch from lysogenic to lytic cycle
AIDS
- The AIDS virus makes DNA on an RNA template
- HIV, the AIDS virus is a retrovirus
- Flow of genetic info is RNA to DNA
- Inside a cell, HIV uses its RNA as a template for making DNA
- The enzyme reverse transcriptase catalyzes reverse transcription
BACTERIA
- Bacteria can transfer DNA in 3 ways
- Bacteria can transfer genes from cell to cell by one of 3 processes
- Transformation: the uptake of foreign DNA from surrounding environment
- Transduction: transfer of bacterial genes by a phage
- Conjugation: union of 2 bacterial cells and the transfer of DNA between them (sex)
DNA
- Once new DNA is in a bacterial cell, part of it may integrate into the recipients chromosome
- Occurs by crossing over between the 2 molecules
- Leaves the recipient with a recombinant chromosome
BACTERIAL PLASMID
- Bacterial plasmids can serve as carriers for gene transfer
- The F factor is a piece of bacterial DNA
- Carries genes for things needed for conjugation
- Contains an origin of replication
- *Can transfer chromosomal DNA by integrating into the door bacterium chromosome or entering the cell as a plasmid
PLASMIDS
- Small circular DNA molecules separate from the bacterial chromosome
- Can serve as carriers for the transfer of genes
