RNA and Protein Synthesis

RNA and Protein Synthesis Section 12-3

Objectives for this section • Compare and contrast DNA and RNA • Name the 3 main types of RNA • Describe transcription and the editing of RNA • Identify the genetic code • Summarize translation • Explain the relationship between genes and proteins

Introduction • The structure of DNA explains how it can be copied, but it does not tell how a gene works. • Genes—coded DNA instructions that control the production of proteins within the cell

Introduction • We now know that the central dogma of biology is DNA to RNA to protein • The first step in decoding the genetic message is to copy part of the DNA nucleotide sequence into RNA (ribonucleic acid) • These RNA molecules contain the coded instructions for making proteins

The Central Dogma of Bilogy

The Structure of RNA • RNA, like DNA, is a long chain of nucleotides • RNA consists of the same components as DNA • A 5-carbon sugar • A phosphate group • A nitrogenous base

The Structure of RNA • There are 3 main differences between DNA and RNA • The sugar in RNA is ribose instead of the deoxyribose in DNA • RNA is generally single-stranded • RNA contains uracil instead of thymine

The Structure of RNA • RNA is like a disposable copy of a segment of DNA • In many cases, RNA is a copy of a single gene—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

Types of RNA • RNA molecules have many functions, but in the majority of cells most RNA molecules are involved in just one job—protein synthesis • There are 3 main types of RNA

Types of RNA • Messenger RNA (mRNA)—carry copies of instructions for assembling amino acids into proteins; serve as “messengers” from DNA to the rest of the cell

Types of RNA • Ribosomal RNA (rRNA)—a component of ribosomes

Types of RNA • Transfer RNA (tRNA)—works during the construction of a protein; transfers each amino acid to the ribosome as it is specified by the coded messages in the mRNA

Types of RNA

Transcription • Transcription—when RNA molecules are produced by copying part of the nucleotide sequence of DNA into a complementary sequence in RNA • Transcription requires the enzyme RNA polymerase—binds to DNA and separates the DNA strands. Then uses one strand of DNA as a template from which nucleotides are assembled into a strand of RNA

Transcription • How does the RNA polymerase know where to start and stop making the RNA copy? • The enzyme will only bind to regions of DNA known as promoters, which have specific base sequences • Similar signals tell the RNA polymerase where to stop

Transcription

RNA Editing • The first molecule of mRNA (known as the pre-mRNA) produced by copying the DNA sequence is like a rough draft and it requires editing • DNA contains sequences of nucleotides called introns, which are not involved in coding for proteins

RNA Editing • The DNA sequences that code for proteins are called exons, because they are expressed in the synthesis of proteins • When an RNA molecule is formed, it contains both introns and exons • The introns are cut out of the RNA molecule while it is still in the nucleus • The remaining exons are spliced back together and form the final pre-mRNA molecule

RNA Editing

The Genetic Code • Remember that proteins are made by joining amino acids into long chains called polypeptides • Each polypeptide contains a combination of any or all of the 20 different amino acids • The properties of proteins are determined by the order in which different amino acids are joined together to produce polypeptides

The Genetic Code • The “language” of mRNA instructions is called the genetic code • RNA contains 4 different nitrogenous bases (U, C, G, A) • How can a code with only 4 letters translate into 20 different amino acids? • The genetic code is read 3 letters at a time, so each “word” of the coded message is 3 bases long

The Genetic Code • Each 3-letter “word” in mRNA is known as a codon • A codon consists of 3 consecutive nucleotides that specify a single amino acid that is to be added to the polypeptide • Example: UCGCACGGU would be read as UCG-CAC-GGU. These codons represent 3 amino acids: serine-histidine-glycine

The Genetic Code • Because there are 4 different bases, there are 64 possible 3-base codons. • Note that some amino acids can be specified by more than one codon • There are also “start” and “stop” codons • Start codons (AUG) tell where protein synthesis is to begin • Stop condons (3 different ones) tell where the end of the polypeptide is

The Genetic Code

Translation • The mRNA molecule has been transcribed and serves as instructions, but we need something to read the instructions and put them to use • In the cell, the ribosome takes care of this • Translation—the decoding of an mRNA message into a polypeptide chain (protein)

Translation Steps in translation • Begins when an mRNA molecule in the cytoplasm attaches to a ribosome • Each codon of the mRNA moves through the ribosome and the proper amino acid is brought to the ribosome by the tRNA

Translation • Each tRNA carries only one kind of amino acid and picks it up based on the anitcodon it is carrying • Example: if the anticodon is UUU, the tRNA would pick up the amino acid with the codon AAA (they are opposites)

What codon would these anticodons pick up?? • ACG • UGG • CAG • GGG • GCC • CAA

Translation • The ribosome forms a peptide bond between the first and second amino acids • At the same time, it breaks the bond with the tRNA molecule and releases it • The ribosome moves on the third amino codon, where a tRNA molecule brings it the amino acid specified by the third codon

Translation • The polypeptide chain continues to grow until the ribosome reaches a stop codon on the mRNA molecule • When the stop codon is reached, it releases the newly formed polypepetide and mRNA molecule, completing the process of translation

Genes and Proteins • What does protein synthesis have to do with the color of a flower, eye color, or height? • Remember that many proteins are enzymes, which catalyze and regulate chemical reactions. • A gene that codes for an enzyme to produces pigment controls flower color. • Proteins are specific tools that build or operate components of living cells.

RNA and Protein Synthesis