Chapter 15: Genes and How They Work

Transcription

The genetic sequence is copied from DNA to mRNA

Translation

mRNA travels out the nucleus to a ribosome to become a protein

mRNA (messenger)

The RNA that copies DNA and brings it to rRNA for protein synthesis

rRNA

Type of RNA that makes up ribosomes, helps catalyze the formation of peptide bonds

tRNA

A type of RNA that carries specific amino acids to the ribosomes and matches them to codons on mRNA

Exon

A segment of a gene that codes for protein

Intron

A segment of a gene that doesn’t code

Poly A Cap

A chain of adenine nucleotides added to the 3’ end of mRNA

One Gene One Enzyme Hypothesis

Each gene directs the synthesis of a single enzyme

Archibald Garrod

First to suggest genes dictate phenotypes through enzymes that catalyze specific reactions in cells; coined the term “inborn errors of metabolism”

Alkaptonuria

Lack of enzymes to break down the acid alkapton; causes black urine, acne, tendonitis, kidney stones, and hunchback

George Beadle and Edward Tatum

Showed that genes specify enzymes

The Central Dogma (Crick)

The fundamental flow of genetic information in living cells: DNA→RNA→Protein

The Genetic Code (Crick and Brenner)

Three bases(one codon) code for one amino acid

The Codon Table (Nirenberg and Khorana)

Read using the 3 bases on the mRNA(right to left): first base on the left, second base on the top, third base on the right

Nucleotide Bases in RNA

Adenine=Uracil, Guanine=Cytosine

The Steps of Both Transcription and Translation

Initiation, Elongation, and Termination

Recognition

RNA polymerase recognizes a promoter sequence in DNA, binds to the strand, and begins reading the gene’s message

Transcription Process (makes mRNA)

As a DNA strand (3’→5’) passes through RNA polymerase, the RNA polymerase builds a single-stranded RNA copy of the gene (mRNA)

Termination of Transcription

The RNA polymerase will encounter a code signaling the end of the gene, releasing the mRNA transcript

Capped and Processed mRNA

mRNA receives a cap and tail after transcription, for protection, recognition, and noncoding sections are removed

Transcription Occurs In

The Nucleus

Promotor Sequence in DNA

TATA Box

Spliceosome

Enzyme that does the editing of the mRNA after transcription

Translation Occurs In

The cytoplasm (in a ribosome)

Initiation in Translation

The “START” sequence AUG in mRNA is recognized by a corresponding tRNA molecule and two ribosomal subunits

Elongation in Translation (makes protein)

As the ribosome moves along the mRNA, amino-acid carring tRNA molecules bind to the next three bases on mRNA, and the tRNA detaches once the corresponding bases are added

Termination in Translation

The ribosome encounters a “STOP” sequence which means the protein assembly is complete, mRNA and protein are released

RNA Start Codon

AUG

RNA Stop Codons

UAG, UAA, UGA

E Site (rRNA)

Where the empty tRNA EXITS

P Site (rRNA)

Holds the tRNA with the growing POLYPEPTIDE chain

A Site (rRNA)

Where tRNA carrying an animo acid ARRIVES

Triplet

A group of 3 DNA nucleotide bases that corresponds to one amino acid

Missense Mutation

A base is changed to another, coding for a different amino acid

Silent Mutation

A base is changed, but it codes for the same amino acid

Frameshift Mutation

A nucleotide is inserted or deleted, which causes a shift in the reading frame

Nonsense mutation

A change in bases that creates a premature stop codon (UAA, UAG, UGA