Transcript Chapter 14
Section 12-3 Chapter 14 Go to Section: Section 12-3 Transcription Adenine (DNA and RNA) Cystosine (DNA and RNA) Guanine(DNA and RNA) Thymine (DNA only) Uracil (RNA only) RNA polymerase DNA RNA Go to Section: Translation Section 12-3 Nucleus Messenger RNA Messenger RNA is transcribed in the nucleus. Phenylalanine tRNA The mRNA then enters the cytoplasm and attaches to a ribosome. Translation begins at AUG, the start codon. Each transfer RNA has an anticodon whose bases are complementary to a codon on the mRNA strand. The ribosome positions the start codon to attract its anticodon, which is part of the tRNA that binds methionine. The ribosome also binds the next codon and its anticodon. Ribosome Go to Section: mRNA Transfer RNA Methionine mRNA Lysine Start codon Mutation - a change in the structure or amount of the genetic material of an organism Nondisjunction - failure of homologous chromosomes to separate properly during meiosis. Results in extra or missing chromosomes Polyploidy - an condition of having more than two sets of chromosomes – plants only transcription factor - an enzyme that is needed to begin and/or continue genetic transcription Intron - a nucleotide sequence that is transcribed from DNA into mRNA but is cut out (not translated). Exon - nucleotide sequences that are transcribed, joined together, and then translated Domain - in proteins, a functional unit that has a distinctive pattern of structural folding 1. What is the origin of genetic differences among organisms? • For the most part, genetic differences among organisms originate as some kind of genetic mutation. 2. What are the causes of mutation? • Mutations occur naturally as accidental changes to DNA or to chromosomes during the cell cycle. • Enzymes repair most DNA that is mismatched during replication, but rarely, some DNA is not repaired. • The rate of mutation can be increased by some environmental factors. Such factors, called mutagens, include many forms of radiation and some kinds of chemicals. 3. What are the effects of mutation? • A small change in DNA may affect one or many amino acid(s) in the protein that results from a gene. • A mutation may have no effect, or may harm or help in some way. • The effect depends on where and when the mutation occurs. 4. What kinds of mutations are possible? • Two main types: gene mutations and chromosomal mutations What kinds of mutations are possible? 1. Gene mutations – (replication errors) a change in the sequence of nucleotides in DNA A point mutation is a change of a single nucleotide in a sequence from one kind of base to another. (substitution) A mutation is silent when it has no effect on a gene’s function. Point mutations are often silent because the genetic code is redundant (each amino acid has multiple codons). *A missense or replacement mutation results when a codon is changed such that the new codon codes for a different amino acid. *A nonsense mutation results when a codon is changed to a “stop” signal. In this case, the resulting string of amino acids may be cut short, and the protein may fail to function. What kinds of mutations are possible? Rarely, errors in replication can cause the insertion or deletion of one or more nucleotides in a sequence. An insertion or deletion can shift the reading frame, or cause a frameshift mutation. In frameshift mutations, the remaining sequence is incorrect. Chromosomal Mutations (meiosis errors) – a change chromosome structure or number A. Errors in the exchange during crossing over in meiosis can cause chromosomal mutations. A deletion occurs when a piece of a chromosome is lost. At the end of meiosis, one of the cells will lack the genes from that missing piece. Such deletions are usually harmful. A duplication occurs when a piece remains attached to its homologous chromosome after meiosis. One chromosome will then carry both alleles for each of the genes in that piece. An inversion occurs when a piece reattaches to its original chromosome, but in a reverse direction. A translocation occurs when a chromosome piece ends up in a completely different, nonhomologous chromosome. Chromosomal Mutations Section 12-4 Deletion Duplication Inversion Translocation Go to Section: 4. What kinds of mutations are possible? B. When homologous chromosomes fail to separate properly during meiosis (nondisjunction), gametes have extra or missing chromosomes. Examples: Autosomal - Down Syndrome - trisomy 21 •Mild to severe mental retardation •Increased frequency of birth defects •Increased susceptibility to disease Sex Chromosome Disorders •Turner’s syndrome (XO): sterile females, sex organs fail to develop normally •Klinefelter’s syndrome (XXY): sterile males, tall stature The Y chromosome has genes necessary for the development of male characteristics. 5. What are the possible effects of mutation in gametes and somatic cells? • Only mutations that occur in gametes can be passed on to offspring, but mutations in body cells affect only the individual in which they occur. • Certain genes control the normal growth, division, and specialization of cells in bodies. – Mutations in these genes can cause a normal somatic cell to “lose control” and begin growing and dividing abnormally. The group of cells that grows will become a tumor. – If the tumor cells begin to invade other parts of the body, they become a form of cancer. • Note that although cancers result from somatic cell mutations, not all somatic cell mutations cause cancer.