Transcription

Learning Targets Transcription is the synthesis of mRNA copied from the DNA base sequences by RNA polymerase

DNA and RNA Compared DNA and RNA are different: Most RNA is single stranded RNA uses uracil in place of thymine RNA uses ribose in place of deoxyribose RNA (ribonucleic acid) is an important molecule required for gene expression to occur. RNA and DNA are very similar molecules, with a few differences. DNA is double-stranded and RNA is single-stranded. DNA has four nucleotides – guanine, cytosine, adenine, and thymine. RNA has four nucleotide too. One is different. RNA has guanine, cytosine, adenine, and uracil. Just as thymine will base-pare with adenine, uracil will base-pair with adenine as well. The 5-carbon sugar in DNA is deoxyribose. In RNA, it is ribose.

Converting a Gene to a Trait Transcription Enzymes (e.g. RNA Polymerase) use the nucleotide sequence of a gene to synthesize a complementary strand of RNA (mRNA). Translation The information carried by mRNA is translated into a sequence of amino acids, resulting in a polypeptide chain that folds into a protein. Gene expression A multistep process including transcription and translation, by which genetic information encoded by a gene is converted into a structural or functional part of a cell or body: a trait. In order for a gene to be expressed, the language of DNA must first be converted to the language of RNA. This is called transcription. An RNA molecule called mRNA (messenger RNA) carries the message from DNA, which is then converted into the language of proteins through the process of translation.

What it’s all about: DNA  RNA  Protein

Replication vs. Transcription

Short video of transcription Here

Transcription: What is the significance of 3’ and 5’? Just like DNA Polymerase enzymes, RNA polymerase can only add new nucleotides to the 3’ end of the growing RNA chain. That is the only place where a dehydration synthesis (condensation) reaction is possible. Transcription looks a great deal like DNA replication. Instead of DNA polymerase adding nucleotide to a growing strand according to the sequence of bases and base-pairing rules on the DNA strand, as in replication - in translation, RNA polymerase adds nucleotides to a growing strand according to the sequence of bases on the DNA strand. Just as in replication, transcription occurs in the 5’ to 3’ direction. The resulting molecule of RNA then separates from the DNA strand.

What would be the resulting mRNA strand from the following DNA? 3’ACTGAGATTACA5’ coding strand 5’TGACTCTAATGT3’ non-coding strand 5’UGACUCUAAUGU3’ mRNA

Post-Transcriptional Modifications and Alternative Splicing 1 2 3 1 2 3 After transcription occurs, the molecule of RNA must be processed before it becomes a fully functional mRNA molecule. There are coding and non-coding regions of the DNA molecule… all of which are transcribed by RNA. The non-coding areas are called exons and the coding areas are called introns. The introns are snipped out of the RNA molecules, leaving only the coding areas behind, which are sealed back together. This process is called splicing. Something interesting that was discovered about the modifications made to RNA molecules is that exons can be spliced together in different orders… essentially making it possible for one gene on the DNA molecule to hold instructions for making more than one protein! This is called alternative splicing. Alternative splicing: one gene = many possible protein products! 1 3 2

Comparing Replication and Transcription What is the final product? DNA RNA Which enzymes are involved? Topoisomerase, Helicase, DNA Polymerase, Ligase RNA polymerase On which end of a nucleotide can another nucleotide be added? 3’ 3’ Which direction does the polymerase move? 5’ to 3’ 5’ to 3’ Where in the cell does this take place? Nucleus Nucleus