From DNA to proteins BioH Chapter 13

The BIG picture – Gametes to genes

DNA - RNA comparison DNA RNA Monomer Sugar Bases used Usual location Characteristic Monomer Sugar Bases used Usual location Function Structure Nucleotide (P-S-B) Deoxyribose A,T,C,G Nucleus only Carries/transfers genetic info Double strand Nucleotide (P-S-B) Ribose A,U,C,G Nucleus AND cytoplasm Carries/transfers genetic info AND PROTEIN SYNTHESIS Single strand Need to know entire chart

Transcription The process of using a DNA template to make RNA Video – start at 2:51, go to 4:47 (Click for video)

Transcription details Enzymes used Helicase – unwind & start strand separation RNA polymerase – brings complementary base-matching nucleotides Ligase – corrections and gap corrections Promoter sequence on mRNA - signals “start” for transcribing DNA sequence into RNA sequence ONE strand only – forming juvenile RNA Uracil used instead of Thymine Use Cytosine, Guanine, Adenine (same as DNA)

Juvenile RNA Immature RNA formed by transcription in nucleus (juvenile RNA, pre-mRNA) Forms mature mRNA at nuclear envelope through use of specific enzymes Most eukaryotic genes contain base sequences that must be removed before translation can occur (introns). They will never be used to form proteins. The RNA genes that remain (exons) form specific proteins that determine traits

Starting Translation NEED solve three issues: Instructions on what specific proteins to build Capture and provide raw material (amino acids) with which to build proteins Place at which to build proteins mRNA tRNA rRNA

Types of RNA mRNA – messenger RNA – carries protein building instructions (very long) tRNA – transfer RNA – picks up amino acid components and delivers them to a ribosome to be assembled into proteins (3 bases long) rRNA – ribosomal RNA – attracts proteins to form a ribosome site for protein synthesis (medium length)

Translation Process of translating mRNA base sequence into proteins

Providing the message - mRNA Contains the coded instructions to make specific proteins (based on the nucleotides’ base sequence) 3 bases as a group (triplet) are called a codon Use the Genetic Code charts to decipher which amino acids are coded by each codon

Genetic Code charts Click on the left picture to review HOW to read the genetic code chart. (video) Use the mRNA codon (transcribed from DNA) to read the charts 64 codons code for only 20 different amino acid building blocks

Supplying raw material - tRNA tRNA attracts amino acids in cytoplasm based “lock and key” structure Matches the codon on the mRNA with its own 3- base anti-codon Delivers specific amino acids to functional ribosome to build polypeptide chain (protein) Watch entire video (Video)

Ribosome Formation - rRNA rRNA combines with other proteins to form the basis of a functional ribosome Small subunit Large subunit Click on ribosome to see how subunits work. See next slide for better description of process. (Video)

Finally – TRANSLATION ! Three stages: Initiation, Elongation & Termination Initiation Mature mRNA leaves nucleus into cytoplasm through nuclear pores Many free amino acids, tRNA and ribosomal subunits floating in cytoplasm Initiator tRNA (attached to the amino acid methionine) attaches to small ribosomal subunit, which then attaches to end of mRNA mRNA moves through ribosome until reaches “start” codon (AUG) on mRNA Large subunit attaches, forming functional ribosome Starts next phase = elongation Initiator tRNA (methionine) has a different structure than elongator tRNA (methionine) and is less common than elongator tRNA

More translation Elongation The initiator tRNA anticodon-mRNA codon bond causes the next bonding site in the ribosome to attract the specific anticodon of another tRNA A 2nd tRNA (with its amino acid) bonds with the 2nd m RNA codon Once this happens, enzymes help form a peptide bond between the two nearby amino acids While other enzymes break the initiator amino acid bond and the first codon-anticodon bond, releasing the intiator tRNA and leaving Methionine to form a peptide bond with the second amino acid The 2nd tRNA moves into the 1st ribosomal bonding site A 3rd tRNA is attracted to the open ribosomal bonding site, allowing the process to continue This process continues until a “STOP” codon is read, initiating the last phase = termination

Translation is terminated Termination The mRNA codon UAA, UAG or AGA (“STOP”) occupies the second ribosomal bonding site No tRNA anticodon bonds with these codons This signals the synthesis process to stop The polypeptide chain (protein) is released from the ribosome The two ribosomal subunits separate We have PROTEINS! Click on the picture to view a video of translation. Multiple ribosomes can attach to a single mRNA forming “polysomes” that can produce multiple proteins from one strand of mRNA. NEXT? Free-floating proteins Golgi apparatus packaging Enter ER for transport

Problems – Gene Mutations Point Mutations – single base-pair change Substitution – one base substituted for the correct one May result in only one amino acid mistake – protein may still function correctly Frameshift Mutations Insertion – extra base inserted Deletion – base is omitted Results in all codons after mutation to be incorrect and may cause protein to malfunction

Problems – Gene Mutations Point mutations – single base changes Deletion – base omitted Insertion – extra base inserted Substitution – one base substituted for the correct one Transposition – DNA segments transposed (exchanged) with another Mutation causes Spontaneous Exposure to mutagens (UV, gamma & X-rays) Natural & synthetic chemicals

Significance and causes of gene mutations Importance Many, if not most, mutations are neutral – causing little or no effect on protein function Can be harmful, causing genetic disorders – cystic fibrosis, sickle cell disease, cancers, HIV tolerance Can be beneficial, evolutionary changes have come about due to positive mutations that allow organisms to better survive their environment Click on apple to watch video about DNA mutations Causes Spontaneous Exposure to mutagens (UV, gamma & X-rays) Natural & synthetic carcinogenic chemicals (video)

The whole process Click to watch overview of DNA transcription and protein assembly