From DNA to Protein Chapter 14

14.1 DNA, RNA, and Gene Expression  What is genetic information and how does a cell use it?

The Nature of Genetic Information  Each strand of DNA consists of a chain of four kinds of nucleotides: A, T, G and C  The sequence of the four bases in the strand is the genetic information

Converting a Gene to an RNA  Transcription Enzymes use the nucleotide sequence of a gene to synthesize a complementary strand of RNA  DNA is transcribed to RNA Most RNA is single stranded RNA uses uracil in place of thymine RNA uses ribose in place of deoxyribose

Fig. 14-3, p. 217 DNA RNA adenine A NH 2 deoxyribonucleic acidribonucleic acid adenine A NH 2 N C N C C N nucleotide base HC C N N N N C CH N C sugar– phosphate backbone guanine G O O N C N C C NH C HC N N NH 2 N CC N CC cytosine C NH 2 cytosine C NH 2 CC HC N N C O C O N N thymine T O base pair uracil U O CCCH 3 C NH HC NH HC C O C O N N DNA has one function: It permanently stores a cell’s genetic information, which is passed to offspring. RNAs have various functions. Some serve as disposable copies of DNA’s genetic message; others are catalytic. Nucleotide bases of DNA Nucleotide bases of RNA

RNA in Protein Synthesis  Messenger RNA (mRNA) Contains information transcribed from DNA  Ribosomal RNA (rRNA) Main component of ribosomes, where polypeptide chains are built  Transfer RNA (tRNA) Delivers amino acids to ribosomes

Converting mRNA to Protein  Translation The information carried by mRNA is decoded into a sequence of amino acids, resulting in a polypeptide chain that folds into a protein  mRNA is translated to protein rRNA and tRNA translate the sequence of base triplets in mRNA into a sequence of amino acids

Gene Expression  A cell’s DNA sequence (genes) contains all the information needed to make the molecules of life  Gene expression A multistep process including transcription and translation, by which genetic information encoded by a gene is converted into a structural or functional part of a cell or body

14.2 Transcription: DNA to RNA  RNA polymerase assembles RNA by linking RNA nucleotides into a chain  A new RNA strand is complementary in sequence to the DNA strand from which it was transcribed

DNA Replication and Transcription  DNA replication and transcription both synthesize new molecules by base-pairing  In transcription, a strand of mRNA is assembled on a DNA template using RNA nucleotides Uracil (U) nucleotides pair with A nucleotides RNA polymerase adds nucleotides to the transcript

Base-Pairing in DNA Synthesis and Transcription

The Process of Transcription  RNA polymerase and regulatory proteins attach to a promoter  RNA polymerase moves over the gene in a 5' to 3' direction, unwinds the DNA helix, reads the base sequence, and joins free RNA nucleotides into a complementary strand of mRNA

Transcription

Fig. 14-5b, p. 219

Animation: Gene transcription details

14.3 RNA and the Genetic Code  Base triplets in an mRNA are words in a protein- building message  Two other classes of RNA (rRNA and tRNA) translate those words into a polypeptide chain

Post-Transcriptional Modifications  In eukaryotes, RNA is modified before it leaves the nucleus as a mature mRNA  Introns Nucleotide sequences that are removed from a new RNA  Exons Sequences that stay in the RNA

Alternative Splicing  After splicing, transcripts are finished with a modified guanine “cap” at the 5' end and a poly- A tail at the 3' end

Animation: Pre-mRNA transcript processing

mRNA – The Messenger  Codon A sequence of three mRNA nucleotides that codes for a specific amino acid

Genetic Information  From DNA to mRNA to amino acid sequence

Genetic Code  Genetic code Consists of 64 mRNA codons (triplets) Some amino acids can be coded by more than one codon  Some codons signal the start or end of a gene AUG (methionine) is a start codon UAA, UAG, and UGA are stop codons

Codons of the Genetic Code

rRNA and tRNA – The Translators  tRNAs deliver amino acids to ribosomes tRNA has an anticodon complementary to an mRNA codon, and a binding site for the amino acid specified by that codon  Ribosomes, which link amino acids into polypeptide chains, consist of two subunits of rRNA and proteins

tRNA

14.4 Translation: RNA to Protein  Translation converts genetic information carried by an mRNA into a new polypeptide chain

Translation  Translation occurs in the cytoplasm of cells  Translation occurs in three stages Initiation Elongation Termination

Initiation  An initiation complex is formed A small ribosomal subunit binds to mRNA The anticodon of initiator tRNA base-pairs with the start codon (AUG) of mRNA A large ribosomal subunit joins the small ribosomal subunit

Elongation  The ribosome assembles a polypeptide chain as it moves along the mRNA Initiator tRNA carries methionine, the first amino acid of the chain The ribosome joins each amino acid to the polypeptide chain with a peptide bond

Termination  When the ribosome encounters a stop codon, polypeptide synthesis ends Release factors bind to the ribosome

Fig c, p. 223 Elongation C An initiator tRNA carries the amino acid methionine, so the first amino acid of the new polypeptide chain will be methionine. A second tRNA binds the second codon of the mRNA (here, that codon is GUG, so the tRNA that binds carries the amino acid valine). A peptide bond forms between the first two amino acids (here, methionine and valine).

Fig d, p. 223 D The first tRNA is released and the ribosome moves to the next codon in the mRNA. A third tRNA binds to the third codon of the mRNA (here, that codon is UUA, so the tRNA carries the amino acid leucine). A peptide bond forms between the second and third amino acids (here, valine and leucine).

Fig e, p. 223 E The second tRNA is released and the ribosome moves to the next codon. A fourth tRNA binds the fourth mRNA codon (here, that codon is GGG, so the tRNA carries the amino acid glycine). A peptide bond forms between the third and fourth amino acids (here, leucine and glycine).

Fig f, p. 223 Termination F Steps d and e are repeated over and over until the ribosome encounters a stop codon in the mRNA. The mRNA transcript and the new polypeptide chain are released from the ribosome. The two ribosomal subunits separate from each other. Translation is now complete. Either the chain will join the pool of proteins in the cytoplasm or it will enter rough ER of the endomembrane system (Section 4.9).

14.5 Mutated Genes and Their Protein Products  If the nucleotide sequence of a gene changes, it may result in an altered gene product, with harmful effects  Mutations Small-scale changes in the nucleotide sequence of a cell’s DNA that alter the genetic code

Common Mutations  Base-pair-substitution May result in a premature stop codon or a different amino acid in a protein product Example: sickle-cell anemia  Deletion or insertion Can cause the reading frame of mRNA codons to shift, changing the genetic message Example: Huntington’s disease

Common Mutations

Animation: Base-pair substitution

What Causes Mutations?  Transposable elements Segments of DNA that can insert themselves anywhere in a chromosomes  Spontaneous mutations Uncorrected errors in DNA replication  Harmful environmental agents Ionizing radiation, UV radiation, chemicals

Mutations Caused by Radiation  Ionizing radiation damages chromosomes, nonionizing (UV) radiation forms thymine dimers

Inherited Mutations  Mutations in somatic cells of sexually reproducing species are not inherited  Mutations in a germ cell or gamete may be inherited

Animation: Protein synthesis summary

Animation: Structure of a ribosome

Animation: Structure of a tRNA

Animation: Transcription

Animation: Uracil-thymine comparison