PROTEIN SYNTHESIS The Blueprint of Life: From DNA to Protein

Figure 7.1 Overview of Replication, Transcription & Translation

Characteristics of RNA RNA is made up of nucleotides  Ribonucleotides RNA contains nitrogenous bases  Adenine  Guanine  Cytosine  Uracil Uracil replaces thymine in RNA RNA usually exists as single- stranded molecule (mRNA, tRNA, rRNA)

Types of RNA RNA is a nitrogenous base that consists of a ribose (5 carbon sugar).  In RNA, the base Uracil is substituted for Thymine in DNA.  The RNA nucleic acid is single stranded. In most cells, RNA molecules are involved in just one job – protein synthesis!  The assembly of amino acids into proteins is controlled by RNA There are 3 main types of RNA: 1. Messenger RNA (mRNA) 2. Ribosomal RNA (rRNA) 3. Transfer RNA (tRNA)

Overview of Gene Expression Genes contain the blueprints for building proteins in cells. RNA is the bridge between DNA and its protein. The “genetic code” is the sequence of bases in DNA that will code for specific amino acids in a growing polypeptide (protein).  There are 20 possible amino acids.

Protein Synthesis Building a protein involves 2 stages:  (1) Transcription; (2) Translation Transcription occurs in the nucleus  DNA  mRNA  Making mRNA from a DNA template in the nucleus. Translation occurs in the cytoplasm at a ribosome  mRNA  protein (using tRNA and rRNA)  Translating instructions in mRNA into a growing polypeptide protein chain at the ribosome.

Overview of Eukaryotic Gene Expression

RNA and Protein Synthesis  TYPES OF RNA  Messenger RNA (mRNA) – carries information from DNA in the nucleus to the ribosomes where the proteins are assembled. It is a partial copy of ONLY the information needed for that specific job. It is read 3 bases at a time – codon.  Ribosomal RNA (rRNA) – found in ribosomes and helps in the attachment of mRNA and in the assembly of proteins.  Transfer RNA (tRNA) – transfers the needed amino acids from the cytoplasm to the ribosome so the proteins dictated by the mRNA can be assembled. (The three exposed bases are complementary to the mRNA and are called the anticodon).

Types of RNA rRNA

DNA → RNA → protein: a diagrammatic overview of information flow in a cell

The Triplet Code

The Codon Chart Using a Codon Chart, we can then see what codon will produce what amino acid. You will have to do this.  Ex. AGG  Ex. CCU  Ex. CCA

RNA DNA RNA polymerase Transcription Adenine (DNA and RNA) Cystosine (DNA and RNA) Guanine(DNA and RNA) Thymine (DNA only) Uracil (RNA only)

The Stages of Transcription: Initiation, Elongation, and Termination

RNA Processing Enzymes in the Eukaryotic nucleus must modify pre- mRNA before the genetic message is dispatched to the cytoplasm. This process is called RNA PROCESSING:  Both ends of the primary transcript are altered.  Addition of 5’ cap and Poly (A) tail  Certain interior sections of the molecule are cut out and then the remaining parts are spliced together – called RNA SPLICING.

RNA Processing: Addition of the 5 cap and poly(A) tail A modified guanosine triphosphate added to the 5’ end. 50 to 250 adenine nucleotides added to the 3’ end created by cleavage downstream of the termination signal. Enzymes modify the two ends of a eukaryotic pre-mRNA molecule. The modified ends help protect the RNA from degradation, and the poly(A) tail may promote the export of mRNA from the nucleus. When mRNA reaches the cytoplasm, the modified ends, in conjunction with certain cytoplasmic proteins, facilitate ribosome attachment. The leader and trailer are not translated, nor is the poly(A) tail.

RNA Processing: RNA splicing The gene shown here and its pre-mRNA transcript have three regions, called exons, that consist mostly of coding sequences; exons are separated by noncoding regions, called introns. During RNA processing, the introns are excised and the exons are spliced together.

Translation Translation is the RNA-directed synthesis of a polypeptide. Generally: it is the reading of the codons on the mRNA strand and the sequencing of them into an amino acid sequence – polypeptide. The Players:  mRNA: already processed within the nucleus, will be the template for the sequence of amino acids.  tRNA: transfers Amino acids from the Cytoplasm to the ribosome.  Ribosome: adds amino acids together from the tRNA and in the sequence of the mRNA.

The Anatomy of a Functioning Ribosome

Translation In response to each codon found on an mRNA strand, tRNA brings the appropriate amino acid to the site of translation Each codon has an anticodon found on a tRNA molecule  The anticodon is a complementary sequence to the codon

Figure The elongation cycle of translation Codon Recognition An incoming tRNA binds to the codon in the A site by a H-bond. Peptide Bond Formation The ribosome catalyzes the formation of a peptide bond between the new amino acid and the growing polypeptide. Translocation The tRNA in the A site is translocated to the P site, taking the mRNA along with it. Meanwhile the tRNA in the P site moves to the E site and is released from the ribosome. The ribosome shifts the mRNA by one codon.

Messenger RNA Messenger RNA is transcribed in the nucleus. Transfer RNA The mRNA then enters the cytoplasm and attaches to a ribosome. Translation begins at AUG, the start codon. Each transfer RNA has an anticodon whose bases are complementary to a codon on the mRNA strand. The ribosome positions the start codon to attract its anticodon, which is part of the tRNA that binds methionine. The ribosome also binds the next codon and its anticodon. mRNA Start codon Ribosome Methionine Phenylalanine tRNA Lysine Nucleus Translation mRNA Go to Section:

Translation Continued

From Polypeptide to Functional Protein During and after its synthesis, a polypeptide chain begins to coil and fold spontaneously, forming a functional protein of specific conformation:  A three-dimensional molecule with secondary and tertiary structure.  A gene determines the primary structure.  The primary structure in turn determines conformation.  In many cases, a chaperone protein helps the polypeptide fold correctly.

Review – PROTEIN SYNTHESIS Transcription  DNA  mRNA (in nucleus)  In eukaryotes will have RNA processing (in nucleus). Translation  mRNA  Polypeptide (at ribosome in cytoplasm) Coiling & Folding  Three dimensional (in cytoplasm as translation is occurring)  Chaperone proteins involved

Mutations Mutations are changes in the DNA sequence that affect genetic information  Gene mutations result form changes in a single gene  Chromosomal mutations involve changes in whole chromosomes Mutations that affect one nucleotide are called point mutations Frameshift mutations are those that shift the “reading” frame of the genetic message by inserting or deleting a nucleotide

Substitution Insertion Deletion Gene Mutations: Substitution, Insertion, and Deletion A substitution point mutation occurs when the wrong base is inserted during DNA replication. An insertion point mutation occurs when an extra base is inserted during DNA replication (causes a frameshift mutation). A deletion point mutation occurs when a base is deleted during DNA replication (causes a frameshift mutation).

Deletion Duplication Inversion Translocation Chromosomal Mutations