RNA & Transcription

The Big Idea Polypeptide

Comparing DNARNA Sugar 5-carbon sugar DeoxyriboseRibose Bases one of four nitrogenous bases G, C, A, ThymineG, C, A, Uracil Structure double strandedsingle stranded

mRNA mRNA: “messenger RNA” Needed to carry the DNA message out of the nucleus You “read” the mRNA 3 bases at a time, called a codon.

codons Sorexthebigreddogranandsatfortheboy s ore xth ebi gre ddo gra nan dsa tfo rth ebo y

tRNA: “transfer RNA” “anticodon” region Specific amino acid at the top, based on the anticodon

tRNA

rRNA Ribosomes have a large and small sub-unit that consist of: rRNA molecules (pink and yellow) Proteins (purple) The green region is the site that catalyzes the formation of peptide bonds.

Review What are the base pairing rules? In DNA, adenine (A) pairs with thymine (T), and guanine (G) pairs with cytosine (C).

Protein functions?

Transcription

Review Questions What is the difference between DNA and genes? DNA is an extremely long, linear molecules containing many genes; genes are relatively short segments of DNA that code for proteins.

What is “gene expression”? Using a portion of DNA to tell the cell how to make a product, (usually a polypeptide).

Central Dogma of Biology DNA RNAPolypeptide TranscriptionTranslation

What is a coding sequence? The sequences of DNA that code for the production of polypeptides. Are non-coding sequences important? YES! Some have functions like to produce tRNA and rRNA. Others regulate gene expression (enhancers & silencers). Telomeres on the ends of chromosomes

RNA synthesis To make mRNA: Catalyzed by RNA polymerase, which pries the DNA strands apart and covalently bonds the RNA nucleotides Follows the same base-pairing rules as DNA, except that in RNA, uracil substitutes for thymine

Synthesis of an RNA Transcript The stages of transcription are Initiation Elongation Termination Figure 17.7 Promoter Transcription unit RNA polymerase Start point Rewound RNA transcript 3 3 Completed RNA transcript Unwound DNA RNA transcript Template strand of DNA DNA 1 Initiation. After RNA polymerase binds to the promoter, the DNA strands unwind, and the polymerase initiates RNA synthesis at the start point on the template strand. 2 Elongation. The polymerase moves downstream, unwinding the DNA and elongating the RNA transcript 5  3. In the wake of transcription, the DNA strands re-form a double helix. 3 Termination. Eventually, the RNA transcript is released, and the polymerase detaches from the DNA.

RNA Polymerase Binding and Initiation of Transcription Promoters are a sequence of DNA near the start of a gene. Promoters signal the initiation of RNA synthesis by being the binding site of RNA polymerase. Transcription factors Help eukaryotic RNA polymerase recognize promoter sequences Figure 17.8 TRANSCRIPTION RNA PROCESSING TRANSLATION DNA Pre-mRNA mRNA Ribosome Polypeptide TATAA A A ATATTTT TATA box Start point Template DNA strand Transcription factors Promoter RNA polymerase II Transcription factors RNA transcript Transcription initiation complex Eukaryotic promoters 1 Several transcription factors 2 Additional transcription factors 3

Elongation of the RNA Strand As RNA polymerase moves along the DNA It continues to untwist the double helix, exposing about 10 to 20 DNA bases at a time for pairing with RNA nucleotides 5’ to 3’

The DNA “antisense” strand acts as a template. The DNA “sense” strand has the code for the protein (same base sequence as the newly formed RNA…but with U for T.)

Terminator region Specific portion of DNA that causes polymerase to detach and transcription to cease.

Transcription Video ated ated

Post-transcriptional Modification Only in Eukaryotic cells. Why? Eukaryotic cells have a nuclear envelope that separates transcription from translation. mRNA can be modified before leaving the nucleus and undergoing translation.

RNA splicing Use two colors to write the phrase below on your tape exactly as you see it. Space the letters so they take up the entire length of the strip of tape. appropriately joined

RNA splicing The segments in one color represent introns. Those in the other color represent exons. Lift up the tape. Working from left to right, cut apart the groups of letters written in the same color and joining the pieces in their original order

Determine from the resulting two strips which strip is made of “introns” and which is made of “exons”. Predict what might happen to a protein if an intron were not removed.

RNA splicing “Alternative Splicing” is a process where one gene codes for several proteins. This is done with multiple exons.

Other post-transcriptional modification Adding a 5’ cap Before transcription is completed Adding a poly-A tail after the transcript has been made.

Regulating Gene Expression

1. By Proteins Example: Repressor proteins that block a promoter.

1. By Proteins The following are regions of DNA that, when protein binds to them, cause the following to occur: Enhancers: Increase the rate of transcription Silencers: decrease the rate of transcription Promoter- proximal elements: proteins must bind to them in order for transcription to be initiated.

2. Epigenetics The influence of the environment on genes. Environmental factors that affect gene expression: Skin pigmentation in relation to sun exposure. Morphogens during embryonic development. Alleles that allow certain protein production only at low temperatures.

2. Epigenetics Modification of nucleosomes is based on environmental factors. Video: ics/intro/ ics/intro/

Nucleosomes help to supercoil the DNA. Supercoiling of DNA bin/netquiz_get.pl?qfooter=/usr/web/home/mhhe/biosci/genbio/maderbiology7/student/olc/art_quizzes/0062fq.htm&afooter=/usr/web/home/mhhe/biosci/genbio/maderbiology7/student/olc/art_quizzes/0062fa.htm&test=/usr/ web/home/mhhe/biosci/genbio/maderbiology7/student/olc/art_quizzes/0062q.txt&answers=/usr/web/home/mhhe/biosci/genbio/maderbiology7/student/olc/art_quizzes/0062a.txt

Supercoiling Animation: =OjPcT1uUZiE

Textbook pg. 348: do the activity “Visualizing Nucleosomes” Use this link (on Haiku): otm.do?momID=7&evtc=Suggest &evta=Moleculeof%20the%20Mo nth&evtl=OtherOptions and click on the two “Discussed structures” at the bottom of the page to manipulate the 3D structures Nucleosomes

Nucleosome

Nucleosome modification Modification to the tails of the histone proteins. Adding an acetyl group Adding a methyl group

Nucleosome modification

Epigenetics Epigenome = the sum of all the epigenetic tags in an organisms’ DNA Different cells have their own methylation pattern that enable cell specialization. “Reprogramming”: Usually epigenetic changes are erased when an egg and sperm meet. “Imprinting”: 1% of the time the epigenome is not erased, but is passed on to the next generation.

Epigenetics So many cool things to learn: