From Gene To Protein

Chapter 17

From Gene to Protein

• The “Central Dogma of Molecular Biology” is DNA RNA protein

• Meaning that our DNA codes our RNA which provides instructions for making protein

• Proteins (you may remember) do many things: structure, support, communication, transportation, enzymes etc.

Transcription and Translation

• Transcription is the synthesis of RNA from DNA

• Translation is the synthesis of a polypeptide (protein) from RNA.

Codons

• Proteins are made of amino acids.

• Each amino acid is coded for by a triplet of nucleotides called a codon.

• For example, AGT = serine

• There are only 20 amino acids, but 64 codons.

Codons

• You must memorize these codons: • AUG = “start” the

process of transcription

• UAA, UAG, and UGA = “stop”

Transcription: DNA to RNA

• First, RNA Polymerase unzips a strand of DNA.

• Transcription can only go from 5’ to 3’

• RNA Polymerase II attaches to DNA at a promoter

• The portion of DNA being transcribed is called a transcription unit

Transcription: DNA to RNA

• The promoter has a region called the TATA box, showing the enzyme where to bind.

Transcription: DNA to RNA

• RNA is now synthesized, as base pairs are added to the unzipped DNA strand.

• RNA is ribonucleic acid. It is a single helix. Instead of T (thymine) RNA has U (uracil).

• So every A in DNA now pairs with U (instead of T).

Transcription: DNA to RNA

• The RNA that is made is called mRNA which stands for messenger RNA.

• Before it leaves the nucleus, it is modified in several ways.• A 5’ cap is added, made of a special

guanine molecule • The 3’ end gets a poly-A tail, a long

chain of 50 to 250 adenines• These help the mRNA bind to ribosomes

RNA splicing

• Some of the RNA isn’t needed to code for proteins, so it is cut out through RNA splicing.

• The non-coding regions that are cut out are called introns, the coding portions the cell needs are called exons.

• Little molecules called small nuclear ribonucleoproteins, snRNA, join with a molecule called a spliceosome to slice the RNA.

Transcription

• http://www.hhmi.org/biointeractive/media/DNAi_transcription_vo1-lg.wmv

Translation: RNA to protein

• The mRNA now leaves the nucleus and binds to a ribosome, where protein synthesis occurs.

• As it passes through the ribosome, tRNA (transfer RNA) molecules, each carrying an amino acid, begin to form a long chain of amino acids.

Translation: RNA to protein

• At one end of tRNA is a triplet code called an anticodon which matches the mRNA.

• At the other end of the tRNA is an amino acid.

Translation: RNA to protein

• The ribosome where this all happens has two pieces, and is made of proteins and RNA called ribosomal RNA (rRNA)

• The subunits are called “large” and “small”

Translation: RNA to protein

• Each ribosome has 3 binding sites for tRNA.

• The “P” site holds the tRNA with the polypeptide going into the big chain

• The “A” site holds the next tRNA in line

• The “E” site is the exit site, once the tRNA drops off its amino acid, it detaches from this site.

• Mutations are changes in the DNA base sequence• These are caused by errors in DNA replication or

by mutagens (a physical or chemical agent such as UV light, x-rays, chemicals)

• Mutagenesis is the creation of mutations

10.16 Mutations can change the meaning

of genes

Translation: RNA to protein

• As the polypeptide chain grows, it begins to coil and fold into its secondary structures (helixes and sheets)

Translation

• http://www.hhmi.org/biointeractive/media/DNAi_translation_vo1-lg.wmv