RNA and Protein Synthesis

R N A RIBONUCLEIC ACID

VOCABULARY gene: sequence of DNA that codes for a protein and thus determines a trait mRNA: RNA molecule that carries copies of instructions for the assembly of amino acids into proteins from DNA to the rest of the cell rRNA: type of RNA that makes up the major part of ribosomes tRNA: type of RNA molecule that transfers amino acids to ribosomes during protein synthesis transcription: process in which part of the nucleotide sequence of DNA is copied into a complementary sequence in RNA RNA polymerase: enzyme similar to DNA polymerase that binds to DNA and separates the DNA strands during transcription

promoter: region of DNA that indicates to an enzyme where to bind to make RNA intron: intervening sequence of DNA; does not code for a protein exon: expressed sequence of DNA; codes for a protein codon: three-nucleotide sequence on messenger RNA that codes for a single amino acid translation: decoding of a mRNA message into a polypeptide chain anticodon: group of three bases on a tRNA molecule that are complementary to an mRNA codon

Types of RNA mRNA rRNA tRNA

TRANSCRIPTION During transcription, RNA polymerase binds to DNA and separates the DNA strands. RNA polymerase then uses one strand of DNA as a template from which nucleotides are assembled into a strand of RNA. RNA polymerase binds only to regions of DNA known as promoters  have specific base sequences  signals in DNA that indicate to the enzyme where to bind to make RNA.

TRANSCRIPTION

RNA Editing Many RNA molecules require a bit of editing before they are ready to go into action. The DNA of eukaryotic genes contains sequences of nucleotides, called introns, that are not involved in coding for proteins. The DNA sequences that code for proteins are called exons because they are “expressed” in the synthesis of proteins. When RNA molecules are formed, both the introns and the exons are copied from the DNA. The introns are cut out of RNA molecules while they are still in the nucleus. The remaining exons are then spliced back together to form the final mRNA.

Many RNA molecules have sections, called introns, edited out of them before they become functional. The remaining pieces, called exons, are spliced together. Then, a cap and tail are added to form the final RNA molecule.

Some RNA molecules may be cut and spliced in different ways in different tissues  makes it possible for a single gene to produce several different forms of RNA. Introns and exons may also play a role in evolution.  makes it possible for very small changes in DNA sequences to have dramatic effects in gene expression

THE GENETIC CODE The Genetic Code The language of mRNA instructions Consist of 20 different amino acids With 64 possible codons  Codon: consist of 3 consecutive nucleotides that specify a specific amino acid (3 bases long)  Proteins are made by joining amino acids into long chains called polypeptides The property of a protein is determined by the order in which different amino acids are joined together to form polypeptides

TRANSLATION Translation  The decoding of an mRNA message into a polypeptide chain (protein)  Takes place on ribosomes in the cytoplasm Begins when mRNA in the cytoplasm attaches to a ribosome Each codon of the mRNA moves through the ribosome Proper amino acid is brought into the ribosome by tRNA Amino acid is transferred to growing polypeptide chain in the ribosome Each tRNA carries only one kind of amino acid Each tRNA has 3 unpaired bases called anticodons which are complementary to one mRNA codon Works like an assembly line Polypeptide chain continues to grow until the ribosome reaches a stop codon Polypeptide chain is released  Protein

TRANSLATION

PROTEIN SYNTHESIS

PROTEINS Protein  Enzymes that catalyze and regulate chemical reactions  Act as microscopic tools to build or operate a component of a living cell  Genes code for proteins that in turn determine genetic traits

MUTATIONS Mutation Changes in the genetic material Effect on organism  Most effects are neutral  Some effects are deadly  Some lead to greater genetic variability in a species Causes  Random mistakes during DNA replication or RNA transcription  Prolonged exposure to excessive radiation or harmful chemicals Types of Mutations  Point mutation: gene mutations involving changes in one or a few nucleotides Occur at a single point in the DNA sequence Include substitutions, insertions and deletions  Substitution: one base is changed to another  Insertions: base is inserted into the DNA sequence  Deletion: Base is deleted from the DNA sequence

Significance of Mutations  Frameshift mutation: mutations that shift the “reading” frame of the genetic message by inserting or deleting a nucleotide May change every amino acid that follows the point of mutation Protein may be altered so much that it is unable to perform its normal function May lead to termination of organism (death)  Chromosomal mutation: involves changes in the number or structure of chromosomes May change the locations of genes on chromosomes DELETIONS: involve the loss or all or part of a chromosome DUPLICATION: produce extra copies of parts of a chromosome INVERSION: reverse the direction of parts of a chromosome TRANSLOCATION: part of one chromosome breaks off and attaches to another Significance of Mutations  Most are neutral  Some are harmful Cause genetic disorders Cause many types of cancer  Some lead to greater genetic variability in a species Polyploidy: organism has extra sets of chromosomes Can cause plants to be stronger and larger

GENE MUTATION TYPES OF MUTATIONS CHROMOSOMAL MUTATIONS