Section 11.2/ 11.3 : From DNA to Protein
*The sequence of the nucleotides in DNA can be converted to an amino acid sequence in proteins. I. Genes & Proteins 1. Genes are the sequence of Nucleotide bases ex) A-C-T-A-G-T, etc.
2. Proteins are produced according to the sequences of bases in the DNA. *Proteins - are complex molecules that become key cell structures and regulators of cell functions. ex) some proteins form muscle fibers some are transport proteins in membranes some are enzymes *enzymes control chemical rxns in a cell, therefore, if DNA makes enzymes, and enzymes control the cell, then DNA must control the cell.
3. It was believed that close to 80,000 genes exist in humans, but now that has been narrowed down to approximately 40,000 - 60,000 genes per human cell. II. RNA (Ribonucleic Acid) A. RNA is also a nucleic acid; but it differs from DNA in three ways: 1. RNA is SINGLE-Stranded; whereas DNA is double-stranded. 2. The sugar in RNA is RIBOSE; not deoxyribose. 3. RNA uses URACIL (U) in place of Thymine (T).
B. The Role of RNA 1. RNA is the “middle man” in protein production; it takes the code from the DNA and uses the code to build the appropriate protein(s). 2. Three Kinds of RNA a) Messenger RNA (mRNA) - messenger RNA is like a mail man. It delivers the message from the DNA out of the nucleus and into the cytoplasm.
b) Ribosomal RNA (rRNA) - ribosomes are made up of ribosomal RNA; their job is to clamp on to the mRNA and read its message so that the appropriate protein can be assembled. c) Transfer RNA (tRNA) - transports amino acids to the ribosome to be assembled into a protein. III. Transcription - From DNA to RNA A. Transcription - process in the nucleus of a cell where enzymes are used to build an RNA molecule copying off of the DNA strand.
*the process of transcription is similar to that of DNA Replication; the difference is in the end result: DNA Replication = 2 copies of DNA double helix Transcription = 1 single stranded piece of RNA, DNA double helix remains the same. B. The Process of Transcription - “The Genetic Code” *the nucleotide sequence transcribed from DNA to a strand of messenger RNA acts as a genetic message, it contains the complete information for the building of proteins.
*the language of proteins uses an alphabet of amino acids; different amino acids put together spell out different proteins. 1. The beginning of Transcription: Transcription begins when a molecule of DNA unzips and unwinds, just as it does in replication.
2. The RNA Nucleotides move in: Once the DNA double helix is unwound and the strands are separated, free-floating RNA nucleotides (A,U,C,G) begin to bond to the “naked” DNA strands.
*Remember that in mRNA, the nitrogenous base Uracil (U) replaces Thymine (T); therefore, everywhere you would normally put a “T” you have to put a “U”instead. *The order of the mRNA bases determine the types of proteins that will be produced. 3. The End of Transcription: *At the end, when the base pairing of RNA nucleotides is complete, the mRNA breaks away from the DNA strand and leaves the nucleus. The original two DNA strands will then bond back together and reform the double helix.
C. Translation: From mRNA to Protein (amino acid) *Translation requires a change in “language” *DNA --> RNA still used the nitrogenous bases; it was the same language: A,C,T,G,U *RNA --> protein, 2 different languages; bases (A,C,T,G,U) to amino acids (glysine, lysine, etc.) 1. What is translation? *Translation is the process of converting the information in a sequence of nitrogenous bases (mRNA) into a sequence of amino acids (which make up a protein).
2. Where does translation happen? *Translation takes place in the ribosomes; most of the time it happens in the ribosomes that are attached to the endoplasmic reticulum.
3. Translating the mRNA code: a) Understanding Codons 1) codons - a distinct set of three nitrogenous bases in the mRNA that code for a specific amino acid. UGG = Tryptophan
2) there are 64 different combinations possible when a sequence of 4 bases are arranged into groups of three: 41 = 4 x 1 = 4 *the first two do not cover all 42 = 4 x 4 = 16 20 amino acids. 43 = 4 x 4 x 4 = 64 *some amino acids can have more than one codon *some codons do not stand for amino acids; they signal “START” or “STOP”
3) The Genetic Code * the messenger RNA (mRNA) genetic code:
*all organisms use the same genetic code, therefore all life on earth evolved form a common origin. Common structure of the amino acid:
b) The First Steps of Translation: 1) The first step: mRNA must bind to a ribosome 2) The ribosome must read the codon sequence “AUG” which signals translation to “START”
4. The Important Job of Transfer RNA (tRNA) *amino acids are found floating in the cytoplasm. In order for them to be assembled into a protein, they must be moved and carried to the correct codon on the mRNA strand. a) tRNA’s job - tRNA is responsible for grabbing the correct amino acid and hooking it up with its corresponding codon on the mRNA strand. b) there is one tRNA for each kind of amino acid c) how does each tRNA know which codon to go to? tRNA has a special end which has an anticodon.
*Anitcodon - a sequence of three nucleotides that pair up with the codon on the mRNA molecule. *each tRNA carries only the amino acid that the anticodon specifies, ex) the tRNA molecule for cysteine has an anticodon of A-C-A which would pair with the mRNA codon U-G-U.
5. The Step-by-Step Process of Translation 1. The starting end of the mRNA strand attaches to a ribosome. 2. A tRNA carrying the appropriate a.a. approaches the ribosome. 3. The anticodon of the tRNA temporarily binds to the codon of the mRNA strand.
4. The ribosome slides along the mRNA strand to read the next codon. 5. The next tRNA comes in and binds its anticodon to the corresponding mRNA codon. 6. An enzyme binds the two a.a. together from the 1st and 2nd tRNA; once this is done, the 1st tRNA is free to leave, but the 2nd tRNA must wait for the next tRNA to come in carrying the next a.a.
1st a.a leaves, next one comes in: 7. This process continues until the ribosome reads a “STOP” codon on the mRNA strand.
Overview: 1) A DNA template is used to make an mRNA strand inside the nucleus. 2) The mRNA strand leaves the nucleus and enters the cytoplasm. 3) Once in the cytoplasm, the mRNA binds to a ribosome and translation begins.
Section 11:3 Genetic Changes *What happens when a DNA sequence gets changed? I. Mutation: A change in the DNA Sequence *Mutation - any change in the DNA sequence A. Mutations in Reproductive Cells: 1. Mutations can affect the reproductive cells - egg & sperm - of an organism by changing the sequence of the bases - A,C,T,G - in the gamete. *If the mutated gamete is used in fertilization, the mutation will present itself in the zygote.
2. Good / Harmless Mutations vs. Bad/ Harmful Mutations a) Harmless Mutations - sometimes mutations in sex cells result in a new trait that is not necessarily detrimental to the organism. ex) no tail in Manx cat or longer necks in giraffes b) Harmful Mutations - many times mutations in sex cells can be hazardous to the health of an organism; it may result in nonfunctional proteins which may cause the zygote to be miscarried or have disorders.
B. Mutations in Body Cells 1. If mutations arise in body cells- bone, tissue, skin, or any other non reproductive cell - the mutation will NOT get passed on to the offspring. *this does not mean that the mutation is not harmful, ex) cancer 2. Any damage to a gene may impair the function of the cell ex) damage to a muscle cell --> no contraction 3. Any damaged cell can pass on its mutation to new cells when it divides. ex) skin cancer
C. Types of Mutations 1. Point Mutations - a change in a single base in the DNA molecule. Usually this occurs when one letter is substituted for another, this may result in a change of an amino acid in the protein. ex) THE DOG BIT THE CAT THE DOG BIT THE CAR *changing a single letter changes the meaning of the sentence.
2. Frameshift Mutations - a mutation in which a single base - A,C,G,T - is either added or deleted, therefore causing a shift in the reading of the mRNA codons. ex) THE DOG BIT THE CAT AND RAN THE DGB ITT HEC ATA NDR AN….. Deletion of “O” THE DOG BIT THE CAT AND RAN THE DOG SBI TTH ECA TAN DRA N…. Addition of “S” *frameshift mutations are generally more harmful than point mutations!
II. Chromosomal Mutations: *Mutations may happen to whole chromosomes, not just a single base in a gene. *Chromosomal Mutations - occurs when parts of a chromosome are broken off and lost or misplaced during mitosis or meiosis; ex) during crossing over A. Effects of Chromosomal Mutations 1. Chromosomal mutations often lead to nondisjunction b/c homologous chromosomes cannot pair correctly when one has extra or missing parts.
Nondisjunction: This could result in gametes that have extra chromosomes or gametes that are missing chromosomes.
2. Most chromosomal mutations are NOT passed on from one generation to the next b/c the zygote usually miscarries. If the zygote does survive it is usually sterile and therefore cannot pass the mutations on to future offspring. B. Types of Chromosomal Mutations 1. Deletion - occurs when part of a chromosome is broken off or lost. 2. Insertion (Duplication) - occurs when part of one chromosome breaks off and attaches itself to its homologous or partner chromosome, causing a repeat of those genes on that chromosome.
3. Inversion - occurs when part of a chromosome breaks off and is reinserted backwards. 4. Translocation - occurs when part of one chromosome breaks off and is added to a different chromosome.
III. Causes of Mutations A. Spontaneous Mutations - mutations that just randomly happen; often occurs as the result of a mistake in base pairing during DNA Replication, or possibly Transcription. B. Mutagens *other mutations are the results of mutagens 1. Mutagens - any agent that causes a change in the DNA sequence.
2. Types of mutagens: a) high energy radiation (UV radiation, X-Rays, nuclear radiation) b) chemicals (asbestos, benzene, cyanide, formaldehyde) c) high temperatures
IV. Repairing DNA *The body has a system of checks and balances, certain repair mechanisms that fix mutations in cells have evolved. *Certain enzymes proofread the DNA and replace incorrect nucleotides w/ correct nucleotides. *These repair mechanisms work well, but are not perfect; the greater the exposure to a mutagen, the higher the chance that a mistake cannot be fixed.