Chapter 13: RNA and Protein Synthesis

Chapter 13: RNA and Protein Synthesis Section 13-1: RNA

The Role of RNA The sequence of bases in DNA carries a code – but what does it mean? DNA contains instructions telling the cell how to make proteins First step to is copy part of the DNA sequence into RNA (ribonucleic acid), which then directs the production of proteins

Comparing RNA and DNA • RNA is also a chain of nucleotides, each containing a 5 carbon sugar, phosphate group, and nitrogenous base • But there are 3 differences, which allow cellular enzymes to tell them apart: • Sugar in RNA is ribose • RNA is single-stranded • RNA contains uracil in place of thymine

Comparing RNA and DNA DNA is like the “master plan” and RNA is the “blueprints” DNA stays in the nucleus, while RNA go to the protein-builders in the cytoplasm - ribosomes

Functions of RNA RNA is a disposable copy of a segment of DNA, the working copy of a single gene It has many functions, but most RNA molecules are involved only in protein synthesis Controls the assembly of amino acids into proteins There are three types of RNA specializing in each aspect of this job

Messenger RNA Carry copies of genes (instructions for proteins) Carry information from DNA in the nucleus to other parts of the cell Abbreviated mRNA

Ribosomal RNA Proteins are assembled at ribosomes, which are made of two subunits The subunits are made of several molecules of ribosomal RNA (rRNA) and up to 80 different proteins

Transfer RNA Transfers amino acids to the ribosome according the code specified by mRNA Abbreviated tRNA

Transcription The process of making mRNA from a DNA template The mRNA is complementary to a template segment of DNA In prokaryotes, RNA synthesis and protein synthesis both take place in the cytoplasm In eukaryotes, RNA is synthesized in the nucleus then moves into the cytoplasm

Transcription - Eukaryotes Requires enzyme RNA polymerase, which binds to DNA during transcription and separates the strands It then uses one of the DNA strands as a template to assemble a complementary RNA strand

Promoters RNA polymerase only binds to promoters, which are regions of DNA with specific base sequences They are signals within the DNA molecule that tell RNA polymerase where to start making RNA There are similar signals in DNA that tell RNA polymerase when to stop transcription

RNA Editing Before RNA molecules become active, sometimes bits and pieces have to be cut out The pieces that are cut out and discarded are called introns In eukaryotes, introns are removed from pre-mRNA before it leaves the nucleus The remaining pieces – called exons – are spliced back together to form the final mRNA

RNA Editing • Biologists don’t fully understand why cells make a large RNA molecule and then throw parts of that molecule away • Some pre-mRNA molecules can be cut and spliced in different ways in different tissues • Introns and exons may also play a role in evolution – small changes in DNA sequences can cause dramatic effects

Animations! http://www.fed.cuhk.edu.hk/~johnson/teaching/genetics/animations/transcription.htm http://www.johnkyrk.com/DNAtranscription.html

Chapter 13: RNA and Protein Synthesis