DNA, RNA & PROTEIN SYNTHESIS CHAPTER 10

DNA = Deoxyribonucleic Acid What is the purpose (function) of DNA? 1. To store and transmit the information that tells cells which proteins to make. 2. To pass on genetic information to our offspring.

There are four different bases: Adenine, Guanine, Cytosine, and Thymine They are represented by their abbreviations: A, G, C, T Adenine and Guanine have 2 carbon rings = Purines Thymine and Cytosine have 1 carbon ring = Pyrimidines

What is the structure of DNA? DNA is a nucleic acid that contains repeating units called NUCLEOTIDES. A nucleotide has 3 parts: A 5 carbon sugar called deoxyribose A nitrogen-containing base A phosphate group – contains the element phosphorus

Watson and Crick Model - proposed that DNA is made up of strands of repeating nucleotide units connected together.

Known as a Double Helix The sides of the ladder equal repeating units of sugar (deoxyribose) and phosphate groups The rungs (crossbars) are made up of the nitrogen bases

The bases on one chain (side) of the DNA COMPLEMENT the bases on the other chain (side) of the DNA The bases are joined together by a HYDROGEN BOND. The bases pair together according to BASE COMPLEMENTARY PAIRING RULES RULE: Cytosine [C] always bonds to Guanine [G]; Adenine [A] always bonds to Thymine [T]

DNA REPLICATION = The process of DNA copying itself  Why does DNA copy itself? To make replicas (copies) of identical cells for growth and repair and to pass on to offspring

STEPS: Strands of the double helix UNWIND. Hydrogen bonds between bases are broken by enzymes called HELICASES. Nucleotides found in the surrounding nucleoplasm join to their base pair by enzymes known as POLYMERASES. Hydrogen bonds form between the bases. When each of the strands is remade, two IDENTICAL strands are made with ONE OLD strand and ONE NEW strand = SEMI- CONSERVATIVE REPLICATION

Let’s Practice What sequence of bases would replicate (attach) with this single strand of opened DNA? A T T G C G T T A T A A C G C A A T

Errors in the making of DNA = MUTATIONS Errors occur at about 10,000/sec for paired nucleotides This number is reduced by enzymes that specialize in proofreading and repairing errors (group of 20+ enzymes) DNA can also be damaged by UV radiation, chemicals, heat, etc.

RNA = Ribonucleic Acid DNA is too large and cannot leave the nucleus; ribosomes the PROTEIN MAKERS are located in the cytoplasm. How can the DNA information get to the ribosomes? Through RNA

Structure of RNA 1. RNA has repeating nucleotides like DNA, but it is only SINGLE stranded. 2. The RNA “backbone” is made of the sugar RIBOSE – not DEOXYRIBOSE like DNA 3. RNA has no base – THYMINE – instead it has the base – URACIL a) In RNA, Uracil [U] not Thymine pairs with Adenine [A] U=A

TYPES OF RNA There are 3 different types of RNA: 1) Messenger RNA (mRNA) – carries message from DNA to the ribosome = single, uncoiled RNA 2) Transfer RNA (tRNA)- hairpin shape, folded single strand of RNA – hooks on to a specific amino acid in the cytoplasm 3) Ribosomal RNA (rRNA) – single strand of RNA in a glob makes up the ribosome. Function unknown.

TRANSCRIPTION = process of DNA giving its message (code) to RNA 1.Steps: a) DNA strand is opened up by enzymes = RNA polymerases b) Nucleotides of RNA are added to the open strand of DNA = TEMPLATE c) U binds to A; C to G d) The strand continues to make RNA until a termination code is reached; The RNA breaks off and is free to leave the nucleus

LET’S PRACTICE If this is one side of a DNA strand; transcribe the other side to make an RNA strand. A T T C A A G A A G C G UA A G U U C U U C G C If you start with a strand of mRNA can you go backwards to make the DNA template? RNA = U U G C G A U C A DNA = A A C G C T A G T

TRANSLATION = process where RNA becomes a protein Genetic Code = genetic information for making a protein is encoded in the bases of the nucleotide 1. Each combination of 3 bases = CODON NOW THAT DNA HAS CHANGED TO RNA HOW DOES IT MAKE A PROTEIN?

Each codon (3 bases) codes for a specific amino acid

Why 3 bases = Codon/Amino Acid? 1) Need 20 different amino acids to make all the proteins needed for survival. 2) 1 base = only 4 amino acids; combination of 2 = 16; combination of 3 = 64 3) Only need 20 amino acids so some are repeated and there is also a START codon (AUG) and 3 stop codons (UAA, UAG, UGA)

Start codon – AUG – methionine engages a ribosome to start translating the mRNA message Stop codons end the making of a particular protein

Steps in making a protein = TRANSLATION 1. mRNA goes to ribosome on ER or in cytoplasm 2. A tRNA molecule hooks on to a 3 base codon of mRNA

How does the tRNA know where to hook on? 1) tRNA has 3 bases = ANTICODON, at its end 2) The anticodon COMPLEMENTS the codon EXAMPLE: If the anticodon has CUA – it will match to the codon GAU

The tRNA molecule has an amino acid that hooks to the top of it Which amino acid hooks onto the tRNA depends on which CODON its anticodon will hook to For example: If anticodon is GCU; codon will be CGA and it will have the amino acid Arginine hook on ANOTHER EXAMPLE

3. Next another tRNA comes in and hooks onto the next mRNA codon (3 bases) in the same manner

4. The amino acids form a bond between them = peptide bond PEPTIDE BOND 5. The first tRNA leaves and another tRNA comes in. 6.This process repeats itself until a stop codon is reached and the protein breaks off

7. The mRNA goes back to the nucleus; the tRNA goes back in cytoplasm to pick up more amino acids. 8.The protein goes one of two places: a) if it was made on a ribosome in the cytoplasm, it stays in the cell b) if it was made on a ribosome in the RER, it goes out of the cell to another cell

The genetic code is UNIVERSAL The codons and the amino acid they code for are the same in ALL organisms So why are all living organisms different? The ORDER of the codons varies causing each organism to make different proteins.