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 Ai_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 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 Ai_translation_vo1-lg.wmvhttp:// Ai_translation_vo1-lg.wmv