Protein Synthesis The Making of Proteins Using Genetic Information

Central Dogma of Molecular Biology Information flows in one direction – from DNA to RNA to proteins.

Relationship between DNA, Genes, Proteins Genetic information passed from parent to offspring is DNA

Each DNA molecule has hundreds of segments known as _genes_. Gene - section of DNA that provides the instructions for making a _protein

 Protein molecules are polymers formed by joining amino acids

 The type of protein made is determined by the order of amino acids.

 1,000’s of different protein molecules are made by the cell every minute

Protein Synthesis - process of using DNA’s code to make proteins.  Two-part process: 1. Transcription 2. Translation

Part 1 Transcription - DNA code used to make RNA Occurs in the cell nucleus. DNA is too big to leave the nucleus. So its “message” is rewritten in the language of RNA molecules which are smaller and can leave the nucleus

How Transcription Works  Enzymes unwind DNA to expose bases in a specific gene. start site nucleotides transcription complex

How Transcription Works RNA polymerase attaches complementary RNA bases to the DNA strand Free nucleotides base pair to the DNA strand RNA polymerase bonds the nucleotides together to form RNA. DNA RNA polymerase moves along the DNA

Transcription – rewriting DNA into RNA

The newly made RNA strand detaches from the DNA after the gene is transcribed. The small RNA strand exits the nucleus through pores in the nuclear membrane RNA

Transcription makes three types of RNA mRNA – Messenger RNA carries DNA’s protein making code to ribosomes rRNA – Ribosomal RNA molecules needed to build ribosomes the cell’s “protein factories” tRNA – Transfer RNA carries amino acids to ribosome to make proteins as directed by the mRNA’s code

8.4 Transcription The transcription process is similar to replication. Transcription and replication both involve complex enzymes and complementary base pairing. The two processes have different end results. –Replication copies all the DNA; transcription copies a gene. –Replication makes one copy; transcription can make many copies. growing RNA strands DNA one gene

8.4 Transcription Translation – mRNA’s code is used to make proteins (polypeptides)

8.4 Transcription Amino acids are coded by mRNA base sequences. A codon is a sequence of three nucleotides on mRNA that codes for an amino acid. codon for methionine (Met) codon for leucine (Leu)

8.4 Transcription The genetic code matches each codon to its amino acid or function. –three stop codons –one start codon, codes for methionine The genetic code matches each RNA codon with its amino acid or function.

8.4 Transcription

 Site of protein synthesis – ribosomes in the cytoplasm.

8.4 Transcription Ribsomes are made from rRNA and proteins

8.4 Transcription Ribosomes consist of two subunits. –The large subunit has three binding sites for tRNA. –The small subunit binds to mRNA.

Transfer RNA (tRNA) carries amino acids to the ribsome. On the other end of tRNA there is a 3 nucleotide sequence called an anticodon An anticodon is a set of three nucleotides that is complementary to an mRNA codon.

8.4 Transcription For translation to begin, tRNA binds to a start codon and signals the ribosome to assemble. –A complementary tRNA molecule binds to the exposed codon, bringing its amino acid close to the first amino acid.

tRNA’s anticodon matches with mRNA’s codon to deliver the correct amino acid needed to make the protein. The Ribosome positions the tRNA molecules close enough so that peptide bonds can form between the amino acids.

8.4 Transcription Once the stop codon is reached, the ribosome releases the protein and disassembles.

Central Dogma

KEY CONCEPT Mutations are changes in DNA that can affect phenotype

What are Mutations?

Mutations are changes to DNA that may be harmful, helpful or have no effect.

Mutations can be caused by several factors. Errors during DNA Replication Mutagens ( environmental factors that mutate genes): - Radiation - Chemicals in environment Errors during Meiosis. Rachel Carson author of Silent Spring

2 Categories of Mutations: 1.Single gene – usually during DNA replication affect one gene 2. Chromosomal – usually occur during meiosis affect many genes

TRANSLOCATION (think “transfer location”) The exchange of DNA segments between nonhomologous chromosomes.

DUPLICATION.. –Gene duplication results from unequal crossing over. Chromosomal mutations may occur during crossing over in Prophase I of Meiosis

NONDISJUNCTION Homologous chromosomes don’t separate in Anaphase I Sister chromatics don’t separate in Anaphase II

Gene Mutations Usually occur in replication Affect one gene and protein made from it

8.4 Transcription Frameshift Mutation A cat sat on my hat Ac ats ato nmy hat Adding or removing bases changes the codon reading frame - this changes the amino acid sequence which changes the protein made.

Ex. Deletion Mutation (shifts the reading frame)

Point Mutation A point mutation substitutes one nucleotide for another. mutated base

Types of Point Mutations

Nonsense point mutation Change codes for a STOP codon Full protein not made NO STOP

Missense Point Mutation * Codes for wrong amino acid * May or harmful or beneficial – depends…

Silent point mutation Change in the DNA sequence that does not change the amino acid sequence or the protein. DNA changes but its expression (phenotype) does not.

Mutations affect the DNA But they may or may not affect phenotype. Chromosomal mutations tend to have a big effect. Some gene mutations change phenotype. – Protein not made – May change protein shape or the active site. blockage no blockage

Mutations in body cells do not affect offspring. Only Mutations in sex cells affect offspring. Natural selection often removes mutant alleles from a population when they are less adaptive.

Transcription is similar to Replication SIMILARITIES Replication (making DNA) andTranscription (making RNA) both * Make a nucleic acid * Occur in the nucleus (eukaryotes) * Occur in all living organisms * Require a number of enzymes to carry out process * Involve complimentary base pariring DIFFERENCES The two processes have different end results. _ Replication produces DNA; Transcription produces RNA – Replication copies all the DNA; transcription copies an individual gene on the DNA. – Replication makes one copy; transcription can make many copies.