RNA Synthesis or “Transcription”

Used in the process of protein synthesis!

RNA (Ribonucleic acid)

• RNA is the nucleic acid that acts as a messenger between DNA and the ribosomes and carries out the process by which proteins are made from amino acids

How RNA is Different from DNA

•Single strand•Linear not helical•Contains uracil instead of thymine•Copy of 1 gene, not entire DNA strand (100’s of genes)•Formed in nucleus prior to protein synthesis•Initially in the nucleus BUT then migrates to cytoplasm via a nuclear pore and eventually attaches to a ribosome

Transcription• The process by which a

gene segment of DNA is copied into a complementary strand of RNA

• The type of RNA formed in this way is called messenger RNA or mRNA

• Occurs so that the information to make a protein can be taken from the nucleus to the ribosome where protein synthesis occurs

There are two other types of RNA:

• Transfer RNA (tRNA): carries amino acids (the building blocks of proteins) to the ribosome where they can be bonded together

• Ribosomal RNA (rRNA) : small organelle (the ribosome) is made of this kind of rRNA and proteins. The ribosome supplies the site where the reaction to link amino acids together occurs.

Lets take a closer look at the steps to make mRNA or transcription:

1) The enzyme RNA polymerase attaches to the DNA strand and unzips it at the gene to be copied

- “unzipping” means that the H-bonds between nitrogenous bases are broken

2) RNA nucleotides base pair with the exposed bases of the gene being copied

- since 2 sides of the DNA strand are exposed, the side being copied is called the “sense” side, while the other uncopied side is called the “missense” side.

3) Sugar-phosphate bonds form between RNA nucleotides

4) Hydrogen bonds break and mRNA is free to leave the nucleus (through a pore) and attach to ribosome in the cytoplasm (protein used in cell) or on RER (proteins used outside cell)

• Finally, the H-bonds reform between the DNA strands and the strands rezip.

• http://www.youtube.com/watch?v=ztPkv7wc3yU&feature=related

A sequence…

• Can refer to either the order the nitrogenous bases come in on a DNA strand or mRNA strand OR the order of the amino acids in a polypeptide chain.

The sequence of nitrogenous bases on an mRNA strand is read in a specific

way…• Each group of 3

nucleotides codes for one amino acid

• This group of 3 nucleotides is called a codon

• There are 20 different amino acids that the bases code for

• Since there are 64 possible combinations of 3 nucleotides, each amino acid is coded for by more than one codon!

• AUG is the “start” codon as it shows the ribosome where to start protein synthesis

• There are 3 “stop” codons that signal the end of protein synthesis.

Closer look at tRNA

• This molecule “reads” the genetic code on the mRNA molecule and supplies the correct amino acids

• One end holds the amino acid, the other contains the “anticodon” that matches up with the codon on the mRNA strand

Anticodon…

• Each tRNA molecule contains 3 nucleotides on one end

• These nucleotides base pair with a codon on mRNA, ensuring that the appropriate amino acid is added to the polypeptide chain

Practice!

• Go to this site and play a couple of the games to practice the steps we’ve learned about DNA replication and protein synthesis so far…

• http://mrhartog.blogspot.com/2009/01/transcription-game-httpwww.html

• Homework: Complete part 3 of the notes!