MBB 201 Lecture Notes - Lecture 13: Deoxyribose, Alternative Splicing, Noncoding Dna
29 May 2018
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Chapter 7: From DNA to Protein: How Cells Read the
Genome
Gene Expression
• Process of how information in a gene is used
From DNA to RNA
Transcription
• Copying the nucleotide sequence in DNA to form RNA
• If the RNA is the final product, translation is not required
RNA
• Linear polymer made from nucleotides and linked through phosphodiester bonds
• Monomers are ribonucleotides
• Contains uracil instead of thymine
• Single stranded
o Can fold into many shapes
o Different shapes allow for various functions (structural, regulatory and catalytic)
Steps of Transcription
• RNA polymerase recognizes transcription start site and unwinds DNA
o Prokaryotes
▪ RNA polymerase (only one type in bacteria) randomly collides with DNA
molecule and slides along
▪ Binds tightly when it finds the promoter region (contains a specific
sequence of nucleotides upstream of starting point)
• Promoter recognized by sigma factor which detects unique
features on the outside of the helix due to the base pairings
• Promoter region has a particular polarity that works to align RNA
polymerase in a particular way so that the correct strand is
transcribed (uses DNA oriented in 3’-5’)
• Which strand is template varies with the gene
▪ Unwinds DNA
o Eukaryotes
▪ Uses three types of RNA polymerase (I, II, and III). Each does a different
type of gene
• I and III- tRNA, rRNA, structural, catalytic
• II- encode for proteins and miRNA (most of the eukaryotic genes)
▪ General transcription factors and polymerase II assemble at promoter
region
• TFIID binds to TATA box usually 25 nucleotides from upstream of
state site. Once TFIID binds RNA polymerase II and other
transcription factors bind to form transcription initiation complex
▪ Phosphate groups are added to tail of RNA polymerase II to release it
form the complex
• TFIIH contains kinases and phosphorylates the tail of RNA
polymerase II
find more resources at oneclass.com
find more resources at oneclass.com
▪ Once transcription has began transcription factors dissociate and start
transcription elsewhere
▪ Protein Phosphatases remove phosphates from RNA polymerase and it
falls off the DNA strand
▪ Non-coding DNA separated by large regions of regulatory DNA
sequences
• One strand acts as template
• Ribonucleotides are added by RNA polymerase
o RNA extends in the 5’-3’ direction
• Elongation continues until RNA polymerase encounters terminator
o Terminator sequence is transcribed into the new RNA
• Strand produced is called the RNA transcript
Differences between Transcription and Replication
• RNA strand does not remain hydrogen bonded to DNA use it elongates
o RNA strand is replaced behind the transcription site and DNA reforms
• RNA much shorter than DNA because it only codes a region
• Many RNAs can be synthesized in a short amount of time
o One can start before another has finished
• Transcription can start without a primer
• RNA polymerase makes more mistakes
Types of RNA
• Messenger (mRNA)- direct synthesis of proteins
o Eukaryotes- mRNA contains information from one gene
o Prokaryotes- adjacent genes get transcribed into one mRNA so contains multiple
proteins
• Ribosomal (rRNA)- form structural and catalytic core of ribosome
• Transfer (tRNA)- select amino acids and hold them in place for the ribosome
• Micro (miRNA)- regulate eukaryotic gene expression
mRNA
• Bacteria
o DNA is exposed to cytoplasm so as RNA is synthesized ribosomes can attach to
free 5’ end of RNA and being translation
o Often does not undergo processing
• Eukaryotes
o DNA is held within the nucleus so it must be transported through the pores in the
nuclear envelope to the cytoplasm before translation begins
o Must undergo RNA processing before it can be transported
▪ Takes places as RNA is synthesized
▪ Enzymes responsible are on the tail of RNA polymerase II
o Splicing- removal of introns by small nuclear RNA (snRNA)
▪ Introns- sequence of noncoding genes
▪ Exons- expressive genes
▪ snRNA is packaged with additional proteins to form small nuclear
ribonucleoproteins (snRNPs) to form core of spliceosome
▪ Alternative splicing- RNA transcript can be spliced in many ways so many
proteins can be produced from one RNA transcript
o RNA Capping- modifies 5’ end (addition of G with a methyl group)
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Chapter 7: from dna to protein: how cells read the. Gene expression: process of how information in a gene is used. Transcription: copying the nucleotide sequence in dna to form rna. If the rna is the final product, translation is not required. Rna: linear polymer made from nucleotides and linked through phosphodiester bonds, monomers are ribonucleotides, contains uracil instead of thymine, single stranded, can fold into many shapes, different shapes allow for various functions (structural, regulatory and catalytic) Ii- encode for proteins and mirna (most of the eukaryotic genes: general transcription factors and polymerase ii assemble at promoter region, tfiid binds to tata box usually 25 nucleotides from upstream of state site. They are broken down by ribonucleases (rnases: lifetime controlled by nucleotide sequences within the mrna, 3"- untranslated region between 3" end and poly-a tail, proteins made in smaller amounts typically have shorter lifetimes as mrna.
