GENE FUNCTION Central dogma of biology - information stored in DNA is copied to RNA, which is used to construct proteins. Chapter 13

A. Comparison of DNA & RNA

B. Transcription Process by which a DNA sequence (gene) is converted to an RNA sequence. Occurs in the nucleus of eukaryotic cells & cytoplasm of prokaryotic cells. Is regulated by operons (bacterial cells) or transcription factors (multicellular organisms). Involves 3 processes: initiation, elongation & termination

1. Initiation RNA polymerase attaches to a promoter on DNA strand. Helicase unzips a short section of DNA. Free RNA nucleotides move in & H-bond to complementary bases on DNA template strand. Promotor is a short section of DNA that marks the start of a gene. Thus, each gene on a chromosome has its own promotor.

2. Elongation 3. Termination RNA polymerase links RNA nucleotides together in a 5’ to 3’ direction. Growing RNA strand peels away from DNA template. 3. Termination RNA polymerase detaches when it reaches a terminator. Terminator is a short section of DNA that marks the end of a gene. Thus, each gene has its own terminator. Completed RNA molecule is released from DNA template.

Usually, several copies of RNA are made at a time. 1 2 3 Determine the base sequence of RNA transcribed from the following DNA template strand. All the resulting RNA molecules are identical. DNA template C A G T A A G C C RNA strand G T C A U U C G G

Three types of RNA are transcribed. mRNA (messenger RNA) - encodes genetic information from DNA & carries it into the cytoplasm. 5’ 3’ codon Examples: AUG codes for amino acid methionine. GGA codes for amino acid glycine. UGU codes for amino acid cysteine. AAG codes for amino acid lysine. CGA codes for amino acid arginine. Each three consecutive mRNA bases forms a genetic code word (codon) that codes for a particular amino acid.

rRNA (ribosomal RNA) - associates with proteins to form ribosomes. large subunit small subunit Subunits are separate in the cytoplasm, but join during protein synthesis (translation).

Amino acid accepting end tRNA (transfer RNA) - transports specific amino acids to ribosome during protein synthesis (translation). Anticodon - specific sequence of 3 nucleotides; complementary to an mRNA codon. Amino acid accepting end Characteristic shape of tRNA is due to fact that some of the bases of tRNA form hydrogen bonds with each other. There are 20 different amino acids. Anticodon sequence on a tRNA determines which of the 20 amino acids it will transport. For example, a tRNA with anticodon sequence UUC will always transport the amino acid phenylalanine (Phe). Anticodon sequence determines the specific amino acid that binds to tRNA.

nucleotide cap is added “poly A tail” is added introns are removed Eukaryotic mRNA must be processed before it exits nucleus & enters cytoplasm. nucleotide cap is added “poly A tail” is added introns are removed If eukaryotic mRNA is not processed, the subsequent step (translation) will result in formation of a nonfunctional protein. Nucleotide cap is a short sequence of modified nucleotides. “Poly A tail” is a series of 100-200 adenines. Introns are non-coding portions of eukaryotic genes. Exons are coding portions of eukaryotic genes. Note: bacterial mRNA is not processed after transcription, and can be translated by ribosomes immediately. Thus, protein synthesis occurs more rapidly in bacterial cells. This is also one of the reasons why we cannot splice a human DNA gene directly into bacterial DNA, forcing bacterial ribosomes to produce human proteins. Bacterial cells can accurately translate human mRNA that has been processed.

C. Translation Process by which an mRNA sequence is translated into an amino acid sequence (polypeptide/protein). Occurs in the cytoplasm of eukaryotic & prokaryotic cells. Requires: mRNA, tRNAs, amino acids & ribosomes. Involves 3 processes: initiation, elongation & termination

1. Initiation Small ribosomal subunit binds to “start codon” [AUG] on mRNA molecule. AUG codon attracts initiator tRNA. The first mRNA codon to specify an amino acid is usually AUG, which attracts an initiator tRNA that carries the amino acid methionine. Thus, methionine signifies the start of a polypeptide/protein sequence. Note: initiator tRNA anticodon (UAC) binds to mRNA start codon AUG.

2. Elongation Large ribosomal subunit binds to small subunit. A second tRNA anticodon binds to the next mRNA codon. A peptide bond forms between the two amino acids.

Initiator tRNA is released. Ribosome moves down mRNA by 1 codon. A third tRNA anticodon binds to the next mRNA codon. Released tRNAs will pick up another amino acid & will be reused. A peptide bond forms between 2nd & 3rd amino acids.

tRNAs continue to add amino acids; polypeptide lengthens.

3. Termination Occurs when ribosome reaches an mRNA stop codon (UGA, UAG or UAA). Stop codons do NOT specify an amino acid. Last tRNA is released, ribosomal subunits separate & new polypeptide/protein is released.

Usually, several copies of the polypeptide/protein are made at a time. 6 5 4 3 2 1 Several ribosomes can translate the same protein from a single mRNA at the same time. Notice that each ribosome has a different-sized polypeptide dangling from it - the closer a ribosome is to the end of a gene, the longer its polypeptide. Chaperone proteins help fold the polypeptide into its characteristic conformation. Alteration of polypeptides: -some must be shortened (ex. Polypeptide that forms insulin is shortened from 80 amino acids to 51). -some must be joined to other polypeptides (ex. Hemoblobin is formed by combining 4 polypeptides). Some polypeptides must be altered before they can function.

Determine the amino acid sequence a ribosome would translate from the following mRNA strand. mRNA C A U G G C U C A A U G A Met Ala Gln STOP Translation usually begins with AUG (start codon), which specifies the amino acid methionine. Ala = alanine Gln = glutamine UGA is a stop codon; it doesn’t specify an amino acid.

Review: Genetic information flows in cell from DNA  RNA  protein. Each gene on DNA codes for production of a specific polypeptide/amino acid.

D. Mutation A physical change in the nucleotide sequence of DNA. May not affect phenotype (silent mutation). Can affect somatic cells (somatic mutation) or sex cells (germinal mutation). Can form spontaneously or be induced by a mutagen. Silent mutations occur because the genetic code is redundant (several codons specify for the same amino acid). Many alterations in the 3rd letter of the codon do not alter the specified amino acid (ex. both CAA & CAG specify the amino acid glutamine). Mutagen - agent that increases the mutation rate (ex. chemicals like thalidomide or acutane; high energy radiation like gamma rays, xrays or UV rays).

1. Point mutation - replacement of one DNA nucleotide with another. missense mutation - point mutation that changes a codon so that a different amino acid is specified. Ex. sickle cell anemia Sickle cell anemia results from a single substitution in the DNA that codes for production of hemoglobin. Adenine is substituted for Thymine - causes Valine to be coded for instead of Glutamine. This substitution is devastating because the abnormal hemoglobin distorts RBC shape & carries much less oxygen.

nonsense mutation - point mutation that changes an amino acid-specifying codon into a stop codon. 2. Frameshift mutation - the insertion or deletion of DNA nucleotides; results in disruption of the reading frame. Ex. cystic fibrosis 3. Expanding repeat - the # of copies of a 3 nucleotide sequence increases over several generations. Ex. myotonic dystrophy Nonsense mutations shorten the polypeptide, which can profoundly affect the phenotype. Frameshift mutations usually have disastrous effects. Cystic fibrosis is caused by the deletion of a single codon. Protein can’t function without missing amino acid.