DNA Replication - Abby & Katie's Study Guides



Chp 10: Molecular Biology of the GeneGene: a discrete unit of hereditary information consisting of a specific nucleotide sequence in DNA (or RNA, in some viruses). Most of the genes of a eukaryote are located in its chromosomal DNA; a few are carried by the DNA of mitochondria and chloroplasts. Gene: portion of a chromosome responsible for the expression of a character Bacteriaphages (or phages for short): viruses that infect bacterial cells10.2 DNA & RNA are Polymers of NucleotidesDNA and RNA are nucleic acids One of the strands of DNA is a DNA polynucleotide, a nucleotide polymer (chain)A Nucleotide is composed Nitrogenous Base 5-Carbon Sugar Phosphate GroupThe nucleotides are joined to one another by a sugar-phosphate backbone Each type of DNA nucleotide has a different Nitrogen-containing base Adenine (A)Cytosine (C)Thymine (T)Guanine (G)RNA (Ribonucleic Acid)- uses sugar ribose (instead of deoxyribose in DNA)- has a Nitrogenous base Uracil (U) instead of ThymineDNA Replication10.5 DNA Replication proceeds in 2 directions at many sites simultaneouslyDNA replication begins at the origins of replication whereDNA unwinds at the origin to produce a “bubble”Replication proceeds in both directions from the origin and Replication ends when products from the bubbles merge with each otherDNA replication occurs in the 5’ to 3’ direction Replication is continuous on the 3’ to 5’ template Replication is discontinuous on the 5’ to 3’ template, forming short segments2 key proteins are involved in DNA replication DNA Ligase joins small fragments into a continuous chain DNA Polymerase Adds nucleotides to a growing chain Proofreads and corrects improper base pairings DNA polymerases and DNA Ligase also repair DNA damaged by radiation and toxic chemicalsDNA replication ensures that all the somatic cells (all cells except for sex cells) in a multicellular organism carry the same genetic information The flow of Genetic info from DNA to RNA to Protein10.6 The DNA genotype is expressed as proteins, which provide the molecular basis for genotypic traitsDNA specifies traits by dictating protein synthesis The molecular chain of command is from DNA in nucleus to RNA and RNA in cytoplasm to protein Transcription: synthesis of RNA under the direction of DNA Translation: synthesis of proteins under the direction of RNA10.7 Genetic info written in codons is translated into amino acid sequencesThe sequence of nucleotides in DNA provides a code for constructing a proteinProtein construction requires a conversion of a nucleotide sequence to an amino acid sequence Transcription rewrites the DNA code intoThe flow of info from gene to protein is based on a Triplet Code: the genetic instructions for the amino acid sequence of a polypeptide chain are written in DNA and RNA as a series of non-overlapping 3-base “words” called codonsTranslation involves switching from the nucleotide “language” to the amino acid “language”Each amino acid is specified by a codon 64 codons are possible Some amino acids have more than one possible codon10.8 The genetic code dictates how codons are translated into amino acidsCharacteristics of the Genetic Code 3 nucleotides specify one amino acid 61 codons correspond to amino acids AUG codes for methionine and signals the start of transcription. This is one of the 61 codons aboveThe genetic code is Redundant, with more than one codon for some amino acid Unambiguous, in that any codon for one amino acid does not code for any other amino acid Nearly universal, the genetic code is shared by organisms from the simplest bacteria to the most complex plants and animals Without punctuation in that codons are adjacent to each other, with no gaps in between10.9 Transcription produces genetic messages in the form of DNAOver view of Transcription:An RNA molecule is transcribed from a DNA template by a process that resembles the synthesis of a DNA strand during DNA replication RNA nucleotides are linked by the transcription enzyme RNA PolymeraseSpecific sequences of nucleotides along the DNA mark where transcription begins and endsThe “start transcribing” signal is a nucleotide sequence called a promotor Transcription begins with initiation as the RNA Polymerase attaches to promotorDuring 2nd phase, elongation, the RNA grows longerAs the RNA reels away, the DNA strands rejoin Finally the 3rd phase, termination, the RNA polymerase reaches a sequence of bases in the DNA template called a Terminator, which signals the end of the geneThe polymerase molecule now detaches from the RNA molecule and the gene10.10Eukaryotic RNA is processed before leaving the nucleus as mRNAMessenger RNA (mRNA)Encodes amino acid sequences Conveys genetic messages from DNA to the translation machinery of the cell, which inProkaryotes occurs in the same place that mRNAEukaryotes, mRNA must exit the nucleus via nuclear pores to enter the cytoplasmEukaryotic mRNA has Introns, interrupting sequences that separate Exons, the coding regionsEukaryotic mRNA undergoes processing before leaving the nucleusRNA splicing removes introns and joins exons to produce a continuous coding sequenceA cap and tail of extra nucleotides are added to the ends of the mRNA to Facilitate the export of the mRNA from the nucleus Protect the mRNA from attack by cellular enzymes Help ribosomes bind to mRNA10.11 Transfer RNA molecules serve as interpreters during translationTransfer RNA (tRNA) molecules function as a language interpreterConverting the genetic message of RNA Into the language of proteins Transfer RNA molecules perform this task by Picking up the appropriate amino acid and Using a special triplet of bases, called an anticodon, to recognize the appropriate codons in the mRNA10.12 Ribosomes build PolypeptidesTranslation occurs on the surface of the ribosome Ribosomes coordinate the functioning of mRNA and tRNA and, ultimately, the synthesis of polypeptidesRibosomes have 2 subunits: small and largeEach subunit is composed of ribosomal RNAs and proteins Ribosomal subunits come together during translation Ribosomes have binding sites for mRNA and tRNAs10.13 An initiation codon marks the start of an mRNA messageTranslation can be divided into the same 3 phases as TranscriptionInitiation Elongation Termination Initiation brings together mRNA A TRNA bearing the first amino acid andThe 2 subunits of a ribosomeInitiation establishes where translation will begin Initiation occurs in 2 stepsAn mRNA molecule binds to a small ribosomal subunit and the first tRNA binds to mRNA at the start codon The start codon reads AUG and codes for Methionine The 1st tRNA has the anticodon UACA large ribosomal subunit joins the small subunit, allowing the ribosome to function The 1st tRNA occupies the P Site, which will hold the growing peptide chain The A Site is available to receive the next tRNA10.14 Elongation adds amino acids to the polypeptide chain until a stop codon terminates translationOnce initiation is completed, amino acids are added one by one to the 1st amino acidElongation is the addition of amino acids to the polypeptide chainEach cycle of elongation has 3 steps:Codon Recognition: the anticodon of an incoming tRNA molecule; carrying its amino acid, pairs with the mRNA codon in the A site of the ribosomePeptide Bond Formation: the new amino acid is joined to the chain Translocation: tRNA is released from the P site and the ribosome makes tRNA from the A site into the P siteElongation continues until the termination stage of translation when: ribosome reaches a stop codon10.15 REVIEW: Flow of genetic info in cell is DNA RNA ProteinTranscription is the synthesis of RNA from a DNA template. In Eukaryotic cells:Transcription occurs in the nucleusThe mRNA travels from nucleus to cytoplasmTranslation = 4 steps. It occurs in the Cytoplasm Amino acid attachment Initiation of polypeptide synthesisElongation Termination 10.6 Mutations can change the meaning of jeansA mutation is any change in the nucleotide sequence of DNA A mutation can involveLarge chromosomal regions or Just a single nucleotide pairMutations within a gene can be divided into 2 categories Base Substitutions involve the replacement of one nucleotide w/ another. Base substitutions may:Have no effect at all, producing a silent mutation Change the amino acid coding producing a Missense mutation, which produces a different amino acidLead to a base substitution that produces an improved protein that enhances the success of the mutant organism and its descendent or Charge an amino acid into a stop codon, producing a nonsense mutation Mutations can result in deletions or insertions that mayAlter the reading frame (triplet grouping) of the mRNA, so that nucleotides are grouped into different codonsLead to significant changes in amino acid sequence downstream of the mutation and Produce a non-functional polypeptide ................
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