Ch Gene  Protein A gene is a sequence of nucleotides that code for a polypeptide (protein) Hundreds-thousands of genes are on a typical chromosome.

The “Central Dogma” of Genetics Genes are coded DNA instructions for the construction of proteins. DNA is located in the nucleus, but proteins are made in ribosomes To avoid damage to the DNA molecules, they are first decoded into RNA which is sent to the ribosome to be the instructions for protein synthesis. DNA  RNA  Protein

DNA v. RNA DNARNA 1.Sugar is deoxyribose 2.Double-stranded 3.A, T, C & G bases 1.Sugar is ribose 2.Single-stranded 3.Uracil instead of thymine

Three types of RNA mRNA (messenger) – carries copies of instructions for assembling amino acids into proteins rRNA (ribosomal) – makes up part of the ribosome tRNA (transfer) – carries each AA needed to build the protein to the ribosome

The Flow of Genetic Information Protein synthesis occurs in 2 steps: transcription (DNA  RNA) & translation (RNA  protein)

Transcription RNA is produced when RNA polymerase copies a sequence of DNA into a complementary RNA strand. DNA : TACGGACACATT RNA : AUGCCUGUGUAA

The Genetic Code

Additional Details of Transcription 1.Initiation: RNA polymerase attached to promoter sequence of DNA and RNA synthesis begins 2. Elongation: RNA elongates and the synthesized RNA strand peels away from DNA template allowing the DNA strands to come back together in regions transcribed 3. Termination : RNA polymerase reaches sequence of DNA bases called a terminator signaling the end of the gene and polymerase molecule detaches

RNA Processing (Editing) Intron- noncoding DNA (“junk”) Exon – coding sequences

Translation mRNA  Polypeptide Decoding of an mRNA message into a polypeptide chain (protein) mRNA molecules are “read” in three base segments called codons Each codon specifies a particular amino acid Some AA are specified by more than one codon.

A Closer Look at tRNA

A Closer Look at Ribsomes

Steps of translation - Initiation After RNA is transcribed in the nucleus, it enters the cytoplasm and attaches to a small ribosomal subunit special initiator tRNA binds to the start codon bringing in the amino acid MET large ribosomal subunit binds to the small one creating a functional ribosome

Steps of Translation - Elongation The anticodon of an incoming tRNA molecule with AA pairs with mRNA codon in A site AA detaches from tRNA in P site and peptide bond forms between it and the AA in the A site translocation – P site tRNA leaves the ribosome and the A site complex (AA, tRNA anticodon and mRNA codon) shift s over to the P site Process continues until a STOP codon is reached

Steps of Translation - Termination Stop codons – UAA, UAG, and UGA do not code for amino acids These codons signal the end of translation The completed polypeptide is released from the last tRNA and exits the ribosome The ribosome splits into individual subunits

Animation

Mutations Mutations are changes in the genetic material – Gene Mutations : change in the nucleotide sequence within a single gene Point mutations are gene mutations involving a change in one or a few nucleotides – Substitution: usually changes only one AA – Frameshift : addition or deletion of a nucleotide shifts the grouping of codons

Substitutions Mutations

Frameshift Mutations

Chromosomal mutations : change in an entire chromosome; may involve loss or duplication of multiple genes 4 Types of Chromosomal Mutations ** Polyploidy = extra set of chromosomes

Causes & Effects of Mutations Causes: Mutagenesis can occur in many ways – Spontaneous mutations occur during DNA replication or recombination – Physical or chemical agents called mutagens may induce mutations (ex. High energy radiation from x-rays or UV light) Effects: Can be harmful, beneficial or neither – May cause of genetic disorders – May be beneficial and lead to production of proteins with new or altered activities, which has an important role in the evolutionary process of natural selection – Some mutations are “silent” and have no effect because the nucleotide change results in a new codon that codes for the same amino acid as the original codon