CH13 – RNA and Protein Synthesis

CH13 – RNA and Protein Synthesis Section 13.1 – RNA SC.912.L.16.5

Key Questions • How does RNA differ from DNA? • How does the cell make RNA?

Vocabulary • RNA • Messenger RNA • Ribosomal RNA • Transfer FNA • Transcription • RNA polymerase • Promoter • Intron • exon

Think About It • Now we know that DNA is the genetic material, and the sequence of the nucleotide bases in its strands must carry some sort of code. For the code to work, the cell has to be able to understand it…so…what exactly do the bases code for and where is the cell’s decoding system?

The Role of RNA How does RNA differ from DNA? When Watson and Crick solved the double-helix structure of DNA they knew how DNA could be copied. All the cell had to do was separate the 2 strands and then use the base-pairing to make a new complementary strand for each. But…

…the structure by itself did not explain how a gene actually works. It took a lot more research to discover that another nucleic acid – ribonucleic acid (RNA), was instrumental in putting the genetic code into action

RNA – like DNA – consists of a long chain of nucleotides. Genes contain coded DNA instructions that tell cells how to build proteins. The first step in decoding these genetic instructions is to copy part of the base sequence from DNA into RNA, which then uses these instructions to direct the production of protein which help to establish an organism’s characteristics.

Comparing RNA and DNA Review: Each DNA nucleotide is made up of : 5 carbon sugar phosphate group nitrogenous base

There are 3 main differences between DNA and RNA: Sugar in RNA is ribose, instead of deoxyribose. RNA is generally a single-strand, not a double- strand RNA contains uracil, instead of thymine. These chemical differences make it easy for enzymes in the cell to tell DNA and RNA apart.

RNA Strand

Master Plans vs. Blueprints • A master plan has all the information needed to construct a building – DNA. • Blueprints are inexpensive, disposable copies of the master plan – RNA. • The DNA molecule stays safely in the cell’s nucleus while the RNA molecules go to the protein-building sites in the cytoplasm – the ribosomes.

Functions of RNA There are 3 types of RNA: Messenger RNA (mRNA) – carries instructions for assembling amino acids into proteins from the DNA and into other parts of the cell. Ribosomal RNA (rRNA) – proteins are assembled on ribosomes, small organelles composed of two subunits which are made up of several dozen proteins and rRNA.

Functions of RNA • Transfer RNA (tRNA) - during the construction of proteins, tRNA transfers each amino acid to the ribosome as it is specified by coded messages in mRNA.

RNA Synthesis • How does the cell make RNA? Cells invest large amounts of raw material and energy into making RNA molecules.

Transcription • Most of the work of making RNA takes place during transcription. • In transcription, segments of DNA serve as templates to produce complementary RNA molecules. • Bases of the transcribed RNA complement the base sequence of the template DNA.

In prokaryotes, RNA synthesis and protein synthesis take place in the cytoplasm.

In eukaryotes, RNA is produced in the cell’s nucleus and then moves to the cytoplasm to play a role in the production of protein.

The ability to copy a single DNA sequence into RNA makes it possible for a single gene to produce hundreds or even thousands of RNA molecules.

The enzyme in RNA transcription is RNA polymerase. RNA polymerase binds to DNA during transcription and separates the DNA strands. It then uses one strand of the DNA as a template from which to assemble nucleotides into a complementary strand of RNA

Promoters • How does RNA polymerase know where to start and stop making a strand of RNA? The answer is that RNA polymerase doesn’t bind to DNA just anywhere. The enzyme binds only to promoters, regions of DNA that have specific base sequences.

RNA Editing • Just like a writer’s first draft, RNA molecules sometimes need a bit of editing before they are ready to be read. • These pre-mRNA molecules have bits and pieces cut out of them before they can go into action.

The cut out, discarded portions are called introns. • In eukaryotes, introns are taken out of the pre-mRNA molecules while they are still in the nucleus. • The remaining pieces, exons, are then spliced back together to form the final mRNA.

Introns and Exons

Promoters are signals in the DNA molecule that show RNA polymerase exactly where to begin making RNA. • There are similar signals in DNA that cause transcription to stop when a new RNA molecule is completed

Why do cells use energy to make a large RNA molecule and then throw away parts? • Biologists aren’t complete sure; however, introns and exons may play a role in evolution, making it possible for very small changes in DNA sequences to have dramatic effects on how genes affect cellular function.

13.1 Assessment (page 365) 1a. Review Describe three main differences between RNA and DNA. RNA – ribose DNA – deoxyribose RNA – single strand DNA – double strand RNA – uracil DNA - thymine

1b. Explain List the three main types of RNA, and explain what they do. mRNA – carries instructions for polypeptide synthesis from DNA in the nucleus to ribosomes in the cytoplasm rRNA – forms an important part of both subunits of a ribosome, where proteins are assembled.

1b. Continued. tRNA carries amino acids to a ribosome and matches them to the coded mRNA message.

1c. Infer Why is it important for a single gene to be able to produce hundreds or thousands of the same RNA molecules? Proteins must be continuously synthesized in the cell, so the instructions coded in genes must be used over and over again. A single gene must be able to produce hundreds or thousands of the same RNA molecules for protein synthesis.

2a. ReviewDescribe what happens during transcription. During transcription, the enzyme RNA polymerase binds to DNA and separates the DNA strands. It then uses one strand of DNA as a template to assemble nucleotides into a complementary strand of RNA.

2b. Predict What do you think would happen if introns were not removed from pre-mRNA? If introns were not removed, the instructions carried by mRNA for assembling amino acids into a protein might be incorrect, and the resulting protein might no function properly.

CH13 – RNA and Protein Synthesis