From DNA to Protein Chapter 14

The Human Code The human genome contains approximately 3 billion base pairs which reside in the 23 pairs of chromosomes within the nucleus of all our cells. Each of the estimated 30,000 genes in the human genome makes an average of three proteins. Humans share 99.9% of DNA with each other (We only differ by .1% genetically!!!!) 98.4% of DNA with Chimpanzees 50% of DNA with Bananas

Impacts, Issues: Ricin and your Ribosomes Ricin (found in castor- oil plant used in plastics, paints, cosmetics) is toxic because it inactivates ribosomes, the organelles which assemble amino acids into proteins, critical to life processes

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

Differences between DNA and RNA Sugar = Deoxyribose Double stranded Bases Cytosine Guanine Adenine Thymine Sugar = Ribose Single Stranded Bases Cytosine Guanine Adenine URACIL (U) These chemical differences make it easy for the enzymes in the cell to tell DNA and RNA apart

Ribonucleotides and Nucleotides

The 3 main types of RNA Messenger RNA (mRNA) Carry a copy of the instructions from the nucleus to other parts of the cell Ribosomal RNA (rRNA) Makes up the structure of ribosomes Transfer RNA (tRNA) Transfers amino acids (proteins) to the ribosomes to be assembled

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

Transcription: DNA to RNA 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 (a specific binding site in DNA close to the start of a gene) 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  Videos: Transcription 3 Minute Transcription/Translation

Transcription

Figure 14.5 Transcription. Fig. 14-5b, p. 219

Transcription Many RNA polymerases can transcribe a gene at the same time

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

Allows one gene to encode different proteins Alternative Splicing Alternative splicing Allows one gene to encode different proteins Some exons are removed from RNA and others are spliced together in various combinations After splicing, transcripts are finished with a modified guanine “cap” at the 5' end and a poly-A tail at the 3' end

mRNA The Messenger mRNA carries protein-building information to ribosomes and tRNA for translation Codon A sequence of three mRNA nucleotides that codes for a specific amino acid The order of codons in mRNA determines the order of amino acids in a polypeptide chain

Consists of 64 mRNA codons (triplets) 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 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

Translation Translation = mRNA to protein Occurs in the cytoplasm on the ribosomes Translation occurs in three stages Initiation Elongation Termination Video: Translation

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

The ribosome assembles a polypeptide chain as it moves along the mRNA 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

Figure 14.12 An example of translation as it occurs in eukaryotic cells. (a, b) In initiation, an mRNA, an intact ribosome, and an initiator tRNA form an initiation complex. (c–e) In elongation, the new polypeptide chain grows as the ribosome catalyzes the formation of peptide bonds between amino acids delivered by tRNAs. (f) In termination, the mRNA and the new polypeptide chain are released, and the ribosome disassembles. Fig. 14-12d, p. 223

Termination When the ribosome encounters a stop codon, polypeptide synthesis ends Release factors bind to the ribosome Enzymes detach the mRNA and polypeptide chain from the ribosome

Polysomes Many ribosomes may simultaneously translate the same mRNA, forming polysomes

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 (frame shift) Can cause the reading frame of mRNA codons to shift, changing the genetic message Example: Huntington’s disease

Sickle cell Mutation

Sickle Cell Mutation The sickle cell mutation is a point mutation that affects the hemoglobin in red blood cells. A single base change from and adenine to uracil changes glutamic acid to valine

Sickle Cell Mutation This mutation however seems to “help” populations where malaria is endemic. (e.g. certain parts of Africa)….. Why? Malaria affects 400 million people annually and kills 2-3 million People with the sickle cell trait are more likely to survive acute malaria illness

Sickle Cell Mutation Red cells affected with malaria become deformed. People with the sickle cell trait remove those cells more affectively which reduce the parasite burden in people

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

McClintock’s Transposable Elements 100 Greatest Discoveries

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

Inherited Mutations Mutations in somatic cells (the cells that are not sperm or eggs) of sexually reproducing species are not inherited Mutations in a germ cell or gamete may be inherited, with evolutionary consequences

Summary: Protein Synthesis in Eukaryotic Cells

Practice 1) Transcribe and translate the following sequence TACCCCAGCGTTACT 2) The DNA strand is mutated to the following: TACCCCAGCGTTACC What type of mutation is this? How does this mutation change the protein? 3) Figure out a mutation in the DNA that will not change the protein.