Introduction to Transcription and Translation

Flow of Information

How does a gene’s encoded message become a trait?

DNA (genotype) proteins phenotype

DNA directs “protein synthesis” – also known as… “gene expression”

Each gene specifies a certain protein.

Basic Principles of Transcription and Translation

RNA (ribonucleic acid) is the link between DNA and protein synthesis. DNA –> RNA –> Protein

Key differences: 1. single strand 2. uses the base U (uracil) instead of

T (thymine)… so U pairs with A 3. has a ribose sugar

Basic Principles of Transcription and Translation

DNA is made of nucleotides. Proteins are made of amino acids. To transition from one language to

the other, 2 major steps are required: transcription and translation.

Basic Principles of Transcription and Translation

Transcription = the synthesis of RNA under the direction of DNA An RNA copy (mRNA) of the DNA carries

the gene’s instructions to the protein-synthesizing machinery (ribosomes)

Translation = the synthesis of a polypeptide, under the direction of RNA Ribosomes facilitate the orderly linking

of amino acids into peptide (protein) chains

Basic Principles of Transcription and Translation

Why an intermediate (RNA)? 1. Protection – original DNA copy stays

inside the nucleus 2. Efficiency – many copies of a protein

can be made simultaneously, and RNA transcripts can be used repeatedly

After the initial transcription of RNA, it must also go through RNA processing.

The Genetic Code

How do 4 nucleotide bases specify 20 amino acids?

Triplicate code: The genetic instructions for a polypeptide

chain are written in the DNA as a series of nonoverlapping, three-nucleotide words.

Each set of 3 bases specifies a certain amino acid.▪ “Codons” = mRNA base triplicates▪ Genetic code was decoded by Marshall Nirenberg

(1961)

The Genetic Code

Only one strand of DNA is used as the template for a given gene.

The mRNA strand is complementary to the DNA.

For a given codon, it is generally understood to be written in the 5’3’ direction, but may be specified

Example: if a DNA strand is 3’-ACC-5’ then the RNA strand would read 5’-UGG-3’.

During translation, the sequence of mRNA codons is decoded, “translated,” into a sequence of amino acids making up a polypeptide chain.

The Genetic Code

Start and Stop codons begin and end translation. AUG = Start

There is redundancy but not ambiguity in the genetic code.

Multiple codons result in the addition of the same amino acid, but a given codon will always result in the addition of the same amino acid.

Cells read codons as three-letter words Reading frame is important

The Genetic Code

The genetic code is nearly universal! Genes from one organism can be

transplanted into another…

The Genetic Code

https://www.youtube.com/watch?v=nHM4UUVHPQM