Lesson 3 – Gene Expression Part 1 - Transcription

Warm-UP Take turns going around your table saying the name of an enzyme or protein involved in DNA Replication, and have someone else say what it does.

Learning Goals Determine the gene to be the molecular unit of heredity. Identify the structure of a gene as a stretch of DNA Determine the first stage of expression as the transcription of DNA to mRNA.

Epidermolysis Bullosa Butterfly Children Genes code for polypeptides. Mutations in genes can cause changes in polypeptide sequence. https://www.youtube.co m/watch?v=Opot60OfjQQ Jonathan Pitre has a genetic disorder known as Epidermolysis Bullosa, that can be traced back to a gene coding for a protein that helps holds skin cells together.

Gene Expression Gene expression refers to the transfer of genetic information from DNA to protein (where it is expressed). To be expressed, DNA is transcribed to RNA and then translated to protein. This theory is called the central dogma of molecular biology. The “Central Dogma of molecular biology” is a phrase coined by Francis Crick, in a sort of tongue-in-cheek manner. It is not seen as strictly true anymore.

A Two Step Process In transcription: A DNA strand serves as a template for the synthesis of an mRNA strand. In translation: An mRNA strand serves as a template for the synthesis of a protein Transcription of a DNA template produces an RNA molecule that is a copy of the genetic information. The nucleotide sequence of this RNA molecule is then translated using the genetic code so that the protein coded for by the gene is produced.

The Structure of a Gene Not every part of a gene is transcribed to RNA, and not all that is transcribed remains in the mature mRNA strand. Read the analogy for the structure of a gene.

RNA Refresher RNA has a different base (Uracil rather than Thymine) and a different sugar (Ribose rather than Deoxyribose). It is more stable as a single stranded molecule. Transcription involves the production of an RNA molecule from a DNA template. RNA is a single- stranded polymer of nucleotides. Recall that it contains the bases Adenine (A), Uracil (U), Guanine (G), and Cytosine (C).

Type of RNA RNA serves more functions in a cell than DNA does, and there are many types based on slight changes in structure and sequence.

Template vs. Coding Strand Remember that DNA is double stranded, and only one strand of DNA will be transcribed. This strand is called the template strand (as it serves as the template for the RNA molecule). The strand that is not transcribed is called the coding strand (as it shares the same code as the RNA molecule). In this example, the top DNA strand is the coding strand and the bottom DNA strand is the template strand. Note how the coding strand shares the same code as the mRNA other than the thymines being replaced by uracils.

Initiation of Transcription RNA Polymerase is an enzyme that catalyzes the synthesis of RNA. Transcription begins when RNA polymerase recognizes a promoter region on the DNA. Once the RNA polymerase complex is bound to the DNA, it unwinds and opens a section of the double helix. The Promoter is just a recognition site that allows Polymerase to bind and open the double helix. It does not get transcribed into mRNA.

Elongation RNA Polymerase pairs complementary NTPs to the template strand, catalyzing the creation of an RNA strand. RNA is more stable as a single strand, and so it falls off, and the DNA reanneals behind it. RNA polymerase can only build the new RNA strand in a 5’ to 3’ direction (the same way DNA polymerase III builds a DNA strand).

Termination of Transcription Specific nucleotide sequences in the DNA template strand serve as a termination signal to transcription. When the RNA Polymerase reaches the signal, it detaches from the DNA strand. The new mRNA strand is released from the transcription assembly, and the DNA double helix reforms and remains unchanged

Review of Transcription Chromosomes contain thousands of genes, and they can be transcribed repeatedly and simultaneously.

mRNA in Eukaryotes Addition of 5’ Cap Addition of 3’ Poly-A tail In Eukaryotes, the mRNA needs to move from the nucleus to the cytoplasm, but must first undergo modification and processing so it is recognized as being useful outside the nucleus. Addition of 5’ Cap Addition of 3’ Poly-A tail Splicing out of intron sequences

5’ Cap A special modified guanosine nucleotide is added to the 5’ end of the RNA transcript so it can be recognized by ribosomes in the cytoplasm.

Poly-A Tail Hundreds of adenine nucleotides are added on the 3’ end of the mRNA molecule. These help with export from the nucleus, and keep the mRNA from being degraded in the cytoplasm. Over time, the tail gets shorter, eventually causing the mRNA to be broken down in the cytoplasm.

Intron Removal Introns (Intervening sequences) are removed from the mRNA strand, and the exons are joined together. This is called splicing, and is regulated by proteins. Exons can be joined together in different ways, meaning one gene can create different mRNA molecules, and thus different polypeptides. https://www.youtube.com/watch?v=itsb2SqR-R0

Review List three differences between DNA and RNA. How is RNA involved in protein synthesis? List and describe the three stages of transcription. Predict where mRNA is transported in the cell after transcription is finished. How do cells synthesize multiple strands of mRNA from one gene simultaneously? Why is this advantageous? Would errors in transcription be more or less damaging than errors in DNA replication? Explain your answer.