DNA Transcription • DNA must be copied to messenger RNA (mRNA) • mRNA goes from nucleus to the ribosomes in cytoplasm • mRNA complements known as codons • Only 3 nucleotide “letters” long • Remember RNA has uracil (U) instead of thymine (T)!
Central Dogma of Molecular Biology • The flow of information in the cell starts at DNA, which replicates to form more DNA. Information is then ‘transcribed” into RNA, and then it is “translated” into protein. The proteins do most of the work in the cell. • Information does not flow in the other direction. This is a molecular version of the incorrectness of “inheritance of acquired characteristics”. Changes in proteins do not affect the DNA in a systematic manner (although they can cause random changes in DNA.
Reverse Transcription • However, a few exceptions to the Central Dogma exist. • Most importantly, some RNA viruses, called “retroviruses” make a DNA copy of themselves using the enzyme reverse transcriptase. The DNA copy incorporates into one of the chromosomes and becomes a permanent feature of the genome. The DNA copy inserted into the genome is called a “provirus”. This represents a flow of information from RNA to DNA. • Closely related to retroviruses are “retrotransposons”, sequences of DNA that make RNA copies of themselves, which then get reverse-transcribed into DNA that inserts into new locations in the genome. Unlike retroviruses, retrotransposons always remain within the cell. They lack genes to make the protein coat that surrounds viruses.
DNA Transcription DNA TRANSCRIPTION Transcription -- When does it occur? RNA TRANSLATION Protein
DNA Transcription RNA differs from DNA 1. RNA has a sugar ribose DNA has a sugar deoxyribose 2. RNA contains uracil (U) DNA has thymine (T) 3. RNA molecule is single-stranded DNA is double-stranded
DNA Transcription RNA – It doesn’t form the helix It is shorter than DNA!!!!
DNA Transcription Principal Points • DNA RNA is transcription • Catalyzed by the enzyme RNA polymerase (does NOT require a primer) • There are similarities and differences in prokaryotic and eukaryotic transcription and the mRNA product REMEMBER: Prokaryotic cells have no nuclei, while eukaryotic cells do have true nuclei.
DNA Transcription STEP 1 - INITIATION RNA polymerase and cofactors bind to DNA and unwind it, creating an initiation bubble. This is a space that grants RNA polymerase access to a single strand of the DNA molecule. Prokaryotes - transcription begins with the binding of RNA polymerase to the promoter in DNA. Eukaryotes - transcription initiation is more complex, a group of proteins called transcription factors mediate the binding of RNA polymerase and the initiation of transcription.
DNA Transcription Step 2: Elongation • One strand of DNA serves as the template for RNA synthesis, but multiple rounds of transcription may occur so that many copies of a gene may be produced. RNA nucleotides line up along one strand of DNA, following the base-pairing rules • The template strand is read 3’ to 5’ - the other strand (5 to 3) is the nontemplate strand. • Just as in DNA synthesis, nucleotides are added to a 3’ hydroxyl (5’ to 3’) Step 3: Termination • Termination is the final step of transcription. Termination results in the release of the newly synthesized mRNA from the elongation complex. Single-stranded messenger RNA peels away and DNA strands rejoin.
DNA Transcription Transcription in Eukaryotes • RNA pol I, located exclusively in the nucleolus, catalyzes the synthesis of 3 of the RNAs found in ribosomes: the 28S, 18S, and 5.8S • RNA pol II, found only in the nucleoplasm, synthesizes mRNAs and some snRNAs. • RNA pol III, found only in the nucleoplasm, synthesizes the tRNAs, 5S rRNA, and snRNAs not made by pol II
DNA Transcription Gene regulatory regions are not necessarily transcribed!
Transcription is divided into 3 steps for both prokaryotes and Eukaryotes—initiation, elongation, and termination. Elongation is highly conserved.
Transcription produces genetic messages in the form of mRNA RNA nucleotide RNApolymerase Direction oftranscription Templatestrand of DNA Newly made RNA Figure 10.9A
DNA Transcription RNA polymerase DNA of gene Promoter DNA Terminator DNA Initiation Elongation Area shownin Figure 10.9A Termination GrowingRNA Completed RNA RNApolymerase Figure 10.9B
DNA Transcription DNA RNA Polymerase pre-mRNA 1. Transcription
DNA Transcription Anatomy of mRNA The mRNA molecule has 3 main parts in both prokaryotes and eukaryotes. Figure 5.8 shows the general structure of the mature, biologically active mRNA as it exists in both prokaryotic and eukaryotic cells.
DNA Transcription What happens to the mRNA? • Carries instructions from DNA to the rest of the ribosome. • Tells the ribosome what kind of protein to make
Types of RNA Genetic information copied from DNA is transferred to 3 types of RNA: __________ RNA: mRNA Copy of information in DNA that is brought to the ribosome and translated into protein by tRNA & rRNA. __________ RNA: rRNA Most of the RNA in cells is associated with structures known as ribosomes, the protein factories of the cells. It is the site of translation where genetic information brought by mRNA is translated into actual proteins. ___________ RNA: tRNA Brings the amino acid to the ribosome that mRNA coded for. From the Virtual Cell Biology Classroom on ScienceProfOnline.com Images: Blueprint, clipart; Factory, Andreas Praefcke; Truck, PRA.
DNA Transcription Types of RNA • Three types of RNA: A. messenger RNA (mRNA) B. transfer RNA (tRNA) C. ribosome RNA (rRNA) • Remember: all produced in the nucleus!
DNA Transcription Prokaryotes versus Eukaryotes – highlighted. LOCATION In prokaryotes (bacteria), transcription occurs in the cytoplasm. Translation of the mRNA into proteins also occurs in the cytoplasm. In eukaryotes, transcription occurs in the cell's nucleus, mRNA then moves to the cytoplasm for translation. REQUIREMENTS DNA in prokaryotes is much more accessible to RNA polymerase than DNA in eukaryotes. Eukaryotic DNA is wrapped around proteins called histones to form structures called nucleosomes. Eukaryotic DNA is packed to form chromatin. While RNA polymerase interacts directly with prokaryotic DNA, other proteins mediate the interaction between RNA polymerase and DNA in eukaryotes. ADDITIONAL STEPS mRNA produced as a result of transcription is not modified in prokaryotic cells. Eukaryotic cells modify mRNA by RNA splicing, 5' end capping, and addition of a polyA tail.
DNA Transcription Question: • What would be the complementary RNA strand for the following DNA sequence? • DNA 5’-GCGTATG-3’
DNA Transcription Summary: