Chapter 13 Biology Notes PDF

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Philip Raven Biology (Period 3) Mrs. Jamerlan 19 February 2013 Biology Chapter 13 Outline I.

Transcription A. During transcription, the first step in protein synthesis, a sequence of nucleotide bases becomes exposed in an unwound region of a DNA strand. The only kind of that sequence acts as a template upon which a single strand of RNA- is synthesized from free nucleotides. 1. The messenger RNA (mRNA) the only kind of RNA that carries protein building codes. 2. Ribosomal RNA (rRNA) becomes part of ribosomes, the structures of which polypeptide chains are assembled. 3. Transfer RNA (tRNA) delivers amino acids one by one to ribosomes, in the order specified by tRNA. 4. An RNA molecule has four kinds of nucleotides. A 5-carbon sugar ribose, a phosphate group, and a base. i. Adenine ii. Guanine iii. Cytosine iv. Uracil. 5. The enzyme RNA polymerase adds the nucleotides one at a time to the end of a growing transcript. Also, transcription results in a single strand of RNA, not two DNA double helixes. 6. Transcription starts after RNA polymerase attaches to its binding site in the DNA, a nucleotide sequence called a promoter. 7. In eukaryotes, other proteins that help with transcription bind to a promoter along with RNA polymerase. Binding to a promoter positions a polymerase at a transcription start site close to a gene. 8. In prokaryotes, transcription and translation occur simultaneously, in the cytoplasm. In eukaryotic cells, transcription occurs in the nucleus, where the resulting RNA is modified before it is shipped to the cytoplasm. 9. Eukaryotic cells tailor their RNA before it leaves the nucleus. 10. Most eukaryotic genes contain one or more introns-nucleotide sequences that are removed from a new RNA. Introns intervene between exons- sequence that stay in the RNA.

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The Genetic Code A. An mRNA is a linear sequence of genetic “words” that are spelled with an alphabet of just four nucleotides. 1. Researchers Marshall Nirenberg, Heinrich Matthaei, Philip Leder, Har Gobind Khorana, and Severo Ochoa revealed the language of mRNA they discovered the correspondence between genes and proteins. 2. An mRNA transcript encodes a sequence of amino acids. To translate its base sequence into an amino acid sequence, you need to know mRNA bases are read as triplets. Each base triplet in mRNA is known as a codon. 3. There are a total of sixty-four codons, many more than are necessary to specify all twenty kinds of amino acids found in proteins. Most amino acids are encoded by more than one codon. 4. The linear order of codons in an mRNA transcript determines the order of amino acids in a polypeptide chain. Some codons signal the start and finish of a gene. 5. The first signal for a translation in most species is AUG. AUG is the codon for methionine, so methionine is the first amino acid in all new polypeptide chains. UAA, UAG, and UGA do not specify an amino acid. They are stop signals that block further additions of amino acids to a new chain. 6. The set of 64 mRNA codons used in protein synthesis is the genetic code. Prokaryotes and some protists of ancient lineages have a few slightly variant codons. tRNA and rRNA A. DNA is the molecule that stores heritable information about building protiens. 1. In eukaryotic cells, the DNA stays protected in the nucleus. mRNAs carry proteinbuilding messages from the DNA to the proteins synthesis machinery in the cytoplasm. 2. The rRNAs are part of ribosomes. 3. In eukaryotic cells, the subunits are assembled in the nucleus and then moved to the cytoplasm, where they converge as intact ribosomes. 4. The tRNAs ferry one amino acid after the next to ribosomes. Each tRNA has two attachment sites. One is an anticodon: a triplet of nucleotides that base-pairs with a complementary mRNA codon. 5. Some tRNAs can base-pair with different codons. In codon-anticodon interactions, the base-pairing rule is loose for the third base in a codon. 6. A ribosome, an mRNA, and tRNAs converge during protein synthesis. The order of codons in the mRNA is the order in which the tRNAs deliver their amino acid cargo to the ribosome. The Three Stages of Translation A. The second part of protein synthesis, translation, is a process that converts genetic information encoded in an mRNA transcript into a new polypeptide chain. Translation occurs in the cytoplasm, and it has three stages : 1. Initiation 2. Elongation

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3. Termination 4. Only initiator tRNAs can begin the initiation stage. The anticodon of an initatior tRNA base-pairs with an AUG codon, and it carries the amino acid methionine. 5. In the elongation stage of translation, a polypeptide chain is assembled as the mRNA threads between the two ribosomal subunits. In elongation, other tRNAs bring amino acids to the ribosome. The tRNAs bind to the ribosome one after the next, in the order determined by the successive codons in the mRNA template. 6. In termination, the mRNA’s stop codon enters the ribosome. No tRNA has an anticodon that base-pairs with this codon, so translation ends. Proteins called release factors bind to the ribosome. 7. Many newly formed polypeptide chains carry out their functions in the cytoplasm. Mutated Genes and their Protein Products A. Gene sequences can change. 1. Small-scale changes in the nucleotide sequence of a DNA molecule are called mutations. Mutations alter the message that becomes encoded in mRNA. Cells have some leeway, because more than one codon can specify the same amino acid. B. Common Gene Mutations 1. During DNA replication, a wrong nucleotide may be linked with an exposed base on a template strand and slip by proofreading and repair enzymes. This mutation is called a base-pair substitution. When the altered message is translated, it may specify a different amino acid or a premature STOP codon. 2. DNA polymerases read base sequence in blocks of three. A deletion is one of the frameshift mutations. These mutations garble the genetic message by shifting the “three-bases-at-a-time” reading frame, and an altered protein results. 3. Frameshift mutations may result from insertions or deletions, in which one or more base pairs are inserted into DNA or deleted from DNA. 4. Another form of insertion mutation is caused by the activity of segments of DNA that can insert themselves anywhere in a chromosome, or transposable elements. 5. Transposable elements can be hundreds or thousands of base pairs long; when one enters a gene sequence, a major insertion that disrupts the gene’s product is the outcome. Transposable elements can occur in all genomes; about 45% of human DNA consists of them or remnants of them. C. How do Mutations Arise? 1. Mutations can happen spontaneously while DNA is being replicated. This is because of the quick pace of the replication (about 20 bases per second in humans and a thousand bases per second in bacteria). 2. DNA polymerases make mistakes at predictable rates, but most fix errors when they occur. 3. Harmful environmental agents can cause mutations. Ionizing radiation can also damage DNA indirectily. It enters living tissue, leaving in its path a trail of destructive damaged DNA.

4. Other forms of radiant energy boost electrons to a higher energy level but not enough to knock them out of an atom. Exposure to UV light can cause two adjacent thymine bases to bond covalently to one another. This bond kinks the DNA. D. The Proof is in the Protein 1. When a mutation arises in a somatic cell of a sexually reproducing individual, its good or bad effects will not endure; it is not passed on to offspring. If it arises in a germ cell or a game, it may enter the evolutionary arena....