DNA, Replication, RNA, Protein Synthesis

REVIEW OF DNA DNA stands for Deoxyribonucleic Acid. DNA contains the genetic code and the “working instructions” for a cell.

Structure of DNA DNA, which is a nucleic acid, is made up of monomers known as nucleotides. Parts of a DNA nucleotide - 5 carbon sugar called Deoxyribose - Phosphate group - Nitrogen Base – Adenine (A), Thymine (T), Guanine (G), Cytosine (C)

Bonding Hydrogen bonds form between the nitrogen bases of two strands. These hydrogen bonds occur between 2 specific nitrogen base pairs: A – T and G – C Memory Helper for base pairing rules: straight letters pair together (A – T) and curved letters pair together (G – C)

DNA Replication DNA replicates during S phase of interphase. This occurs before cell division (mitosis). DNA replicates in the nucleus, while in the form of chromatin. DNA replicates according to Chargaff’s base pairing rules : A – T, G – C

DNA Replication – The Process Replication takes place in a 2 Step Process: 1. An enzyme breaks the hydrogen bond that is between the nitrogen bases, unzipping DNA. The DNA molecule is unzipped in several places ~ like a “broken zipper”.

2. After DNA is unzipped, a second enzyme, called DNA polymerase helps bind free nucleotides to the exposed nitrogen bases. They bind according to Chargaff’s rules, so A – T, G – C. This continues along both strands of DNA in both directions.

The end result is 2 identical DNA molecules. Each new DNA molecule consists of one original strand and one newly formed strand. These 2 DNA molecules are the sister chromatids that undergo mitosis. Remember this???

RNA RNA stands for Ribonucleic acid RNA carries out the instructions of DNA by protein synthesis. DNA is too large (double stranded) to leave the nucleus, so RNA (which carries the code of DNA), leaves the nucleus through the nuclear pores and help make proteins at ribosomes out in the cytoplasm.

Structure 5 carbon sugar called ribose Phosphate group RNA, which is a nucleic acid, is made up of monomers known as nucleotides. Parts of a RNA nucleotide 5 carbon sugar called ribose Phosphate group Nitrogen Base – Adenine (A), Uracil (U), Guanine (G), Cytosine (C). RNA is a single helix molecule. Nucleotide

Compare & Contrast of DNA & RNA Characteristic DNA RNA Sugar Deoxyribose Ribose Base Thymine Uracil Structure Double Helix Single Helix

Types of RNA There are 3 forms of RNA involved in carrying out the genetic instructions of DNA: 1. mRNA – messenger RNA; It carries the instructions (message) from DNA in the nucleus to the Ribosomes. 2. tRNA – transfer RNA; It carries (transfers) the amino acids to the ribosome according to the message of mRNA. 3. rRNA – ribosomal RNA; It is part of the structural component of the ribosomes; it also produces enzymes needed to bond the amino acids together to form the proteins.

Protein Synthesis Protein synthesis occurs in 2 steps: The genetic material contained in DNA must be transcribed (rewritten) onto an RNA molecule. The message contained in RNA must be translated (converted) into a specific molecule.

Transcription (DNA transferred to RNA) Transcription takes place in the nucleus and allows for the genetic code of DNA to be carried out to the ribosome. This happens in G1 of interphase. (Remember G1 where normal cell activities take place?)

Steps of Transcription 1. The enzyme, RNA polymerase, unzips part of the DNA molecule.

2. RNA polymerase adds nucleotides (A,U,G,C) according to Chargaff’s rules (A – U, G – C), and RNA is synthesized.

Initially mRNA has too many nucleotides, some code for proteins and some do not. Nucleotides in mRNA are classified as introns or exons. Introns – not required for protein synthesis – they are removed from mRNA. Exons – required for protein synthesis – kept and bonded together once the introns are removed. “EX”ONS = “EX” pressed “IN”TRONS = “IN” between the good stuff

3. Then the new mRNA strand breaks apart from DNA, leaves the nucleus through the nuclear pores and goes to the ribosomes, the site of protein synthesis.

There are 2 important ways that transcription differs from replication: Only one side of the DNA molecule is copied in transcription, both sides are copied in replication. In transcription, RNA is used, so the nitrogen base that pairs with adenine is uracil, in DNA replication, adenine pairs with thymine.

http://www.johnkyrk.com/DNAtranscription.html RNA being made! - transcription

Translation (RNA transferred to Proteins) Translation takes place in the cytoplasm at ribosomes. The message is carried by mRNA, it is interpreted by tRNA, and the final product is a protein.

Codon Codon = a group of 3 nucleotides on mRNA that codes for a specific amino acid. A codon can be made up of any combination of the 4 nucleotides (A, U, G, C), therefore there are up to 64 possible codons. (b/c 4 x 4 x 4 = 64) Ex: AAA, AAU, AAG, AAC Many different codons represent the same amino acid b/c there are 64 possible codons, and only 20 amino acids. Amino acids are the building blocks (or monomers) of proteins. Even though there are only 20 amino acids, there are many, many proteins. It is the number and arrangement of the amino acids that makes every protein unique.

Start codon: AUG – starts the making of the amino acid chain. Stop codon: signal to stop the making of the amino acid chain. Genetic Code is universal, therefore the codons codes for the same amino acid in all organisms.

The mRNA message is translated by the ribosome using these codons. 1st letter 3rd letter 2nd letter

Practice: AUG _________ UCA _________ GAA _________ AAU _________ CGG _________ GAG _________ GAA _________ AAU _________

tRNA The tRNA molecule (which is out in the cytoplasm) has an amino acid on one end and an anticodon on the other end.

Anticodon – an anticodon is a group of 3 nucleotides on tRNA that pairs up to the codon on mRNA. Acts as a binding site - The anticodon binds to the mRNA codon according to base pairing rules (A-U, G-C) and insures that the proper amino acid is brought to the ribosome. tRNA - transfers the amino acids specified by the mRNA codon to the inside of the ribosomes for protein synthesis. The amino acid is then transferred to the growing polyopeptide chain. (polypeptide chain = becomes a protein)

Building of a protein The mRNA molecule slides through the ribosome one codon at a time. The specified amino acids are carried to the ribosome by tRNA. As another tRNA arrives at the ribosome, the tRNA already there is bumped off, but leaves its amino acid behind. This process continues and a chain of amino acids forms until a stop codon is reached. Then the ribosome releases the amino acid chain, which coils and folds to form a protein.

Steps for translation: mRNA attaches to the ribosome mRNA message in the form of codons is read by the ribosome. Amino acids are made as the mRNA is being read tRNA transfers these amino acids to the inside of the Amino acid chains form in the ribosome Stop codon is reached Protein (amino acid chain) is released and tRNA and mRNA is released.

http://learn.genetics.utah.edu/units/basics/transcribe/ Suggested Study Questions Pp 315 (3-10, 12-16, 18-21, 23)