Ch. 10.6-16 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

DNA RNA

1. Sugar is deoxyribose

2. Double-stranded3. A, T, C & G bases

1. Sugar is ribose2. Single-stranded3. 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 Intron- noncoding DNA (“junk”)DNA (“junk”)Exon – coding Exon – coding sequencessequences

TranslationmRNA 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 genes4 Types of Chromosomal Mutations

** ** PolyploidyPolyploidy = extra set of = extra set of chromosomeschromosomes

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