DNA, RNA, and Protein Synthesis

DNA, RNA, and Protein Synthesis Chapter 10

Discovery of DNA • 1928- Fredrick Griffith • He found that when harmless bacteria are mixed with dead harmful bacteria, the harmless will absorb the genetic material of the harmful and become harmful themselves • Transfer of genetic material is called transformation

Discovery of DNA • 1940s- Avery and colleagues • Wanted to know what caused transformation (DNA, RNA, or protein) • They separated these individual parts and tested them. • They found DNA was the cause of transformation • In other words, they found if harmless bacteria took in harmful bacteria’s DNA, the harmless became harmful.

Discovery of DNA • 1952- Hershey and Chase • Wanted to test whether DNA or protein was the genetic material that viruses pass on when they infect an organism. • They used viruses that infect bacteria (called bacteriophages) • They radioactively labeled the DNA and the protein (this allowed them to trace the path of each) • They found DNA was injected into the bacteria to infect it, not protein. So DNA was the genetic material

Discovery of DNA • 1950s- Watson, Crick, Franklin, and Wilkins • Franklin and Wilkins discover DNA is helical • Watson and Crick build a model of DNA and determine it is a double helix

DNA Structure • Made of nucleotides (so nucleotides are the monomers of DNA!) • Nucleotides have 3 parts: • Nitrogenous base (there are 4 kinds) • Phosphate Group • 5 carbon sugar called deoxyribose

One Strand of DNA phosphate deoxyribose bases nucleotide

DNA Structure • Nitrogenous bases: • Contain nitrogen and is a base • Purines- (double ringed) • Adenine (A) • Guanine (G) • Pyrimidine's- (single ringed) • Cytosine (C) • Thymine (T)

DNA Structure • DNA is made up of 2 straight chains of nucleotides • The bases on each of those chains are attracted to each other and form hydrogen bonds • The force of thousands or millions of hydrogen bonds keeps the two strands of DNA held tightly together

DNA models • Since the sugar-phosphate “backbone” of DNA never change, we often simplify DNA into the letters of the nitrogenous bases. • For example, this DNA strand can be simplified to… TGAC ACTG

DNA Structure • Base pairing rulesin DNA: • Hydrogen bonds form between specific pairs • Adenine ALWAYS pairs with Thymine • Cytosine ALWAYS pairs with Guanine • These pairs (A-T and C-G) are called complementary base pairs • Each complimentary pair contains one single and one double ringed base

DNA Structure • Because of the base pairing rules, one strand of DNA is complementary to the other strand (otherwise they would not stick together!) • The order of the nitrogenous bases on DNA is called its base sequence • So if one strand has a base sequence of TGCC, the other strand will have ACGG.

Let’s Practice • Write the complimentary strand for… TGACCGAT TGGCCAATATA

END DAY 1

DNA Replication • DNA Replication is the process by which DNA is copied in a cell before the cell divides.

DNA Replication • Step 1: Enzymes called Helicases separate the two strands of DNA • Hydrogen bonds are broken • Replication fork: Y-shaped region formed as bonds are broken

DNA Replication • Step 2: Enzymes called DNA polymerases add nucleotides to the separated strands • Nucleotides are found floating free in the nucleus • The addition of new nucleotides occurs in opposite directions addition towards the replication fork addition away from the replication fork

DNA Replication • In Eukaryotes, several replication forks form and replication continues until all of the DNA has been replicated. • If only 1 was formed it would take too long to replicate DNA (53 days for humans!!) • Once replication is complete, 2 DNA molecules exist. • Each molecule is made from one old strand and one new • This is called semi-conservative replicationsince each new strand has kept one of the original strands.

DNA Replication • Replication is usually very accurate • There is only about 1 error for every BILLION nucleotides added! • The reason is that DNA Polymerases also “proof-reads” the DNA and fixes any errors during replication

DNA Replication • If an error does occur, it results in a different nucleotide sequence in the new DNA strands calledamutation • A change in even one nucleotide can be very harmful to an organism • Some mutations can affect the growth of cells, causing growth to accelerate, resulting in cancer • Changes can be good - mutations sometimes lead to adaptations and therefore evolution

Protein Synthesis • DNA is the “code” for hereditary characteristics. • The genetic code is how organisms store hereditary information which is first translated into amino acids • DNA codes for all of the body’s proteins (like enzymes)

Protein Synthesis • Genes are sequences located in DNA that code for specific characteristics • The code (or gene) for the production of the protein melanin is in your DNA and creates your hair and skin color

Protein Synthesis • The “code” or “recipe” within DNA cannot be read directly- • DNA cannot leave the nucleus and proteins are made in the cytoplasm of cells • So the code is transcribed (copied) and translated (turned into something useful) by ribonucleic acid (RNA) However,

Protein Synthesis • Remember, proteins make us who we are • Responsible for • chemical reactions • hereditary characteristics (such as eye color) • Recall the monomers of proteins are amino acids • DNA holds the recipe for the amino acid sequence of all the proteins we make

DNA vs RNA • Both are made of nucleotides • Both are involved in protein synthesis • DNA has the sugar deoxyribose, while RNA has the sugar ribose • RNA uses the nitrogenous base uracil (U) instead of thymine (T) used in DNA • RNA is single stranded, while DNA is double stranded • RNA is usually MUCH shorter than DNA

END DAY 2

Protein Synthesis • There are three major types of RNA • Messenger RNA (mRNA) – carries the genetic instructions from the DNA to the ribosomes

Protein Synthesis • Ribosomal RNA (rRNA) – part of the structure of ribosome • Remember ribosomes make proteins mRNA

Protein Synthesis Amino Acid • Transfer RNA (tRNA) – transfers the amino acids to the ribosomes to make proteins Anticodon

Protein Synthesis

Protein Synthesis Transcription • The first step in protein synthesis is transcription: • RNA polymerase (enzyme) binds to a genes promoter region • A promoter is just a specific nucleotide sequence where the RNA polymerase can attach • The RNA attaches to the RNA polymerase and the DNA begins to uncoil

Transcription cont. • The RNA polymerase adds complimentary nucleotides resulting in a straight chain RNA molecule • The DNA code determines what bases will be added • Example if the DNA code for a gene is ATCCGTT, then the RNA will be UAGGCAA • Remember, RNA does not have Thymine, it has Uracil!!

Transcription cont. • The copying of DNA continues until the RNA polymerase reaches a termination signal • That is a specific sequence of nucleotides that tells the RNA polymerase to “STOP” and release the RNA and DNA • The RNA is called mRNA, because it is the messenger of the “code” from the DNA to the ribosomes

END DAY 3

Protein Synthesis - Translation • Once the newly made RNA leaves the nucleus it attaches to a ribosome at the promoter region. • Ribosomes will “read” 3 nucleotides in the RNA code at a time • These 3 nucleotides are called codons. • Each Codon codes for • START signal • Amino Acid • STOP signal

Translation cont. • Example, the sequence AUG codes for the amino acid Methionine a START signal (This is the only start signal) • ALL mRNA molecules start with AUG, otherwise, they would not have a start region for protein synthesis

Translation cont. • So, in translation, the RNA is translated into amino acids, which are put together to form proteins (or polypeptides) • The translation occurs with the help of tRNA, which carries the amino acids

Translation cont. • When the ribosome reads the start sequence (AUG), a tRNA molecule comes along with the anticodon • The anticodon is the complementary sequence. • The complementary bases bond with each other and the amino acid methionine begins the protein synthesis within the ribosome • So, tRNA transfers amino acids to the ribosome

Amino Acids • Recall there are only 20 amino acids • Most amino acids have more than one codon • Example, Leucine’s codons are UUA, UUG, CUU, CUC, CUA, and CUG • But each codon codes for ONLY 1 amino acid • Example, CUU only codes for Leucine and nothing else

Translation cont. • After the start sequence, the ribosome moves to the next codon. • Let’s say the next codon is GUC • Now a tRNA that has the anticodon for that codon attaches to the ribosome carrying the amino acid Valine. • The amino acid Valine attaches to the Methionine from before (now we have a dipeptide!)

Translation cont. • This process continues and the polypeptide grows until the STOP codon is reached • UAA, UAG, and UGA are the only stop codons • The protein, ribosome and all RNA is released to perform other needed functions

Protein Synthesis - Overview • Amino Acids are listed by their CODON!!! • That would be the 3 nucleotide sequence on the mRNA

Protein Synthesis - Overview • Now use the CODON chart to figure out the amino acid sequence

Protein Synthesis - Overview • 1 – Methionine (start) • 2 - Threonine • 3 – Glutamic acid • 4 - Leucine • 5 - Arginine • 6 - Serine • 7 - STOP

Some Great Resources • http://nobelprize.org/educational_games/medicine/dna_double_helix/index.html • DNA games

DNA, RNA, and Protein Synthesis