From Gene To Protein

Chapter 17

The Connection Between Genes and Proteins

• Proteins - link between genotype (what DNA says) and phenotype (physical expression)

• Beadle and Tatum – 1st to make connection between genes and enzymes that carry out genes through bread mold experiments.

http://fig.cox.miami.edu/~cmallery/150/gene/17x3.jpg

• Bridge between DNA, proteins - RNA.

• RNA similar to DNA - sugar ribose; contains uracil instead of thymine.

• RNA single-stranded.

http://gibk26.bse.kyutech.ac.jp/jouhou/image/nucleic/rna/rna_bb_st.gif

• Nucleotides found in DNA and RNA - code - determines order of amino acids.

• 2 steps - transcription and translation.

http://www.ktf-split.hr/glossary/image/nucleotide.gif

• Transcription - DNA serves as template for complementary RNA strand.

• Transcription produces mRNA strand (messenger RNA).

• Translation uses mRNA sequence to determine order of amino acids - creates polypeptide.

http://www.brooklyn.cuny.edu/bc/ahp/BioInfo/graphics/Transcription.02.GIF

• Bacteria - transcription and translation occur at once.

• Eukaryotes, most transcription occurs in nucleus, translation occurs at ribosome.

• Before primary transcript can leave nucleus - modified during RNA processing before enters cytoplasm.

• Genetic code consists of triplet code - series 3 nitrogen bases that code for specific amino acid.

• 64 possible combinations of nitrogen bases.

• Only 20 amino acids = each amino acid has more than 1 code.

http://www.dls.ym.edu.tw/lesson/gen.files/codon.jpg

• 61 of 64 codes specific to an amino acid.

• Other 3 - stop codons - determine when process stops.

• Specific code that signals start of translation - also codes for amino acid.

• Start begins correct reading frame of polypeptide.

• Transcription, 1 DNA strand - template strand, provides template for ordering sequence of nucleotides in RNA transcript.

• Translation, blocks of 3 nucleotides, codons, decoded into sequence of amino acids.

• Possible to take genetic code of 1 organism, place it into another - nearly universal.

• Firefly gene for luminescence transplanted into tobacco plant.

• Bacteria can be inserted with specific genes to synthesize genes in large amounts.

Synthesis and Processing of RNA

• mRNA transcribed from template of original gene.

• RNA polymerase separates DNA strands and bonds RNA bases along complementary strand.

• Bases can only be added to 3’ end.

http://www.csu.edu.au/faculty/health/biomed/subjects/molbol/images/7_9.jpg

• Specific sequences determine where transcription starts and where it ends.

• Promoter – initiates; terminator ends.

• 3 stages in transcription.

• Presence of promotor sequence determines which strand of DNA helix is template.

• Proteins (transcription factors) recognize promotor region, especially a TATA box, bind to promotor.

• After they have bound to promotor,RNA polymerase binds to transcription factors to create transcription initiation complex.

• RNA polymerase starts transcription.

• RNA polymerase moves along - nucleotides added to 3’ end.

• Single gene can be transcribed simultaneously by several RNA polymerases at a time.

• Growing strand of RNA trails off from each polymerase.

• RNA splicing - removal of large portion of RNA molecule.

• Most eukaryotic genes and RNA transcripts have long noncoding stretches of nucleotides.

• Noncoding segments, introns, lie between coding regions.

• Coding regions - exons.

http://ghs.gresham.k12.or.us/science/ps/sci/ibbio/chem/nucleic/chpt15/introndeletion.gif

• RNA splicing removes introns, joins exons to create mRNA molecule with continuous coding sequence.

• Splicing done by spliceosome.

The Synthesis of Proteins

• Translation - cell interprets series of codons along mRNA molecule.

• Transfer RNA (tRNA) transfers amino acids from cytoplasm’s pool to ribosome.

• Ribosome adds each amino acid carried by tRNA to growing end of polypeptide chain.

• Translation - tRNA links mRNA codon with appropriate amino acid.

• tRNA arriving at ribosome carries specific amino acid at 1 end, has specific nucleotide triplet, anticodon, at other.

• Anticodon base-pairs with complementary codon on mRNA.

http://bioweb.uwlax.edu/GenWeb/Molecular/Theory/Translation/ribosome.jpg

• tRNA synthesized like other forms of RNA.

• Once in cytoplasm, each tRNA used repeatedly to pick up and drop off that amino acid.

• Anticodons of tRNA recognize more than one codon.

• Rules for base pairing between 3rd base of codon and anticodon relaxed (wobble).

http://www.geneticengineering.org/chemis/Chemis-NucleicAcid/Graphics/tRNA.jpg

• Each ribosome has binding site for mRNA 3 binding sites for tRNA molecules.

•P site holds tRNA carrying growing polypeptide chain.

•A site carries tRNA with next amino acid.

• Discharged tRNAs leave ribosome at E site.

• Translation divided into 3 stages: initiation

elongation termination

http://nobelprize.org/educational_games/medicine/dna/a/translation/pics/translation2.gif

• Initiation brings together mRNA, tRNA with 1st amino acid, 2 ribosomal subunits.

• Elongation - series of 3-step cycles as each amino acid added to previous one.

• 3 steps of elongation continue codon by codon to add amino acids until polypeptide chain completed.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin CummingsFig. 17.18

• Termination - 1 of 3 stop codons reaches A site.

• Release factor binds to stop codon, hydrolyzes bond between polypeptide and tRNA in P site.

• Frees polypeptide, translation complex disassembles.

• 2 types of ribosomes active in process.

• 1Free ribosomes suspended in cytosoll synthesize proteins in cytosol.

• 2Bound ribosomes attached to cytosolic side of endoplasmic reticulum.

Fig. 17.21

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

• Bacteria and eukaryotes have differences in cellular machinery and in details of processes.

• Eukaryotic RNA polymerases differ from prokaryotic; require transcription factors.

http://www.personal.psu.edu/faculty/r/c/rch8/workmg/TxnPromotersCh11_files/image052.png

• Differ in how transcription terminated.

• Ribosomes also different. • Prokaryotes can transcribe and

translate same gene simultaneously.

• Mutations - changes in genetic material of cell (or virus).

• Chemical change in 1 base pair of gene causes point mutation.

• Occur in gametes or cells producing gametes - may be transmitted to future generations.

http://staff.jccc.net/PDECELL/evolution/mutations/mutypes.gif

• Point mutation that results in replacement of pair of complementary nucleotides with another nucleotide pair - base-pair substitution.

• Some base-pair substitutions have little or no impact on protein function (silent mutations).

http://fajerpc.magnet.fsu.edu/Education/2010/Lectures/26_DNA_Transcription_files/image008.jpg

• Other base-pair substitutions cause readily detectable change in protein.

• Missense mutations - still code for amino acid, change indicated amino acid.

• Nonsense mutations change amino acid codon into stop codon, nearly always leading to nonfunctional protein.

Fig. 17.24

Copyright © Pearson Education, Inc., publishing as Benjamin Cummings

• Insertions and deletions - additions or losses of nucleotide pairs in gene.

• Unless these mutations occur in multiples of 3 - cause frameshift mutation.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 17.24

• Mutations can occur in number of ways. Errors can occur during DNA replication, DNA repair, or DNA recombination.

• Mutagens - chemical or physical agents that interact with DNA to cause mutations.

• Physical agents - high-energy radiation like X-rays and ultraviolet light.