DNA Deoxyribonucleic acid DNA is a nucleic acid

DNA

• Deoxyribonucleic acid (DNA) is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms & some viruses. • The main role of DNA molecules is the long-term storage of information. DNA is often compared to a set of blueprints or a recipe, since it contains instructions needed to construct other components of cells, such as proteins & RNA molecules. The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in regulating the use of this genetic information.

DNA • Genes are made of DNA = deoxyribonucleic acid • How could DNA code for everything genes do? – Genes had to carry information from one generation to the next – Genes had to put that information to work by determining the inheritable characteristics of organisms – Genes had to be easily copied, because it is replicated every time the cell divides

DNA’s Structure • A long molecule made of units called nucleotides • Each nucleotide is made of 3 basic parts: – A 5 -carbon sugar called deoxyribose – A phosphate group – A nitrogenous base (There are 4 kinds…) A (Adenine) G (Guanine) Purines T (Thymine) C (Cytosine) Pyrimidines

A Single DNA Nucleotide Phosphate Group Deoxyribose Sugar Nitrogenous Base

te Phospha Group DNA Structure Nitrogenous Base Weak Hydrogen Bonds Deoxyribose Sugar

Figure 12– 7 Structure of DNA Section 12 -1 Nucleotide Hydrogen bonds Sugar-phosphate backbone Key Adenine (A) Thymine (T) Cytosine (C) Guanine (G) Go to Section:

The Chargaff “rule” • It was discovered that the percentages of guanines (G) and cytosines (C) are almost equal and the percentages of adenines (A) and thymines (T) are almost equal • Erwin Chargaff came up with a “rule” that guanine (G) pairs with cytosine (C) and adenine (A) pairs with thymine (T) (DNA Base Pairing)

Let’s pair up DNA! • What would pair up with ATCG? TAGC • What would pair up with ATTAGC? TAATCG • What would pair up with ATACGGTC? TATGCCAG

The Double Helix • Rosaline Franklin’s x-ray showed that two DNA strands are twisted around each other. • James Watson and Francis Crick used Franklin’s x-ray to make a model of DNA. • Watson and Crick’s model of DNA was a double helix, in which two strands were wound around each other. • Watson and Crick also discovered that hydrogen bonding held the nitrogenous bases together.

• The two strands of DNA unwind or “unzip” breaking the hydrogen bonds and separating. Then each strand becomes the guide or “template” for the making of a new strand. • A protein called an enzyme called DNA polymerase breaks the nitrogen base bonds and the two strands of DNA separate, polymerizes individual nucleotides to produce DNA and “proof reads” the new DNA.

STEP 1 Replication fork • Two original strands of DNA separates by unwinding • This occurs through the action of an enzyme that breaks the hydrogen bonds between the strands • The two areas on either end where the DNA separates is the REPLICATION FORK

STEP 2 • At the replication fork, DNA polymerase moves along the DNA Strands adding nucleotides • As DNA polymerase moves along two strands of DNA form

Step 3 • DNA Polymerase continues until all the nucleotides have been added • Two new identical molecules of DNA are formed • Another enzyme connects all the pieces of DNA together

Proofreading • At the end of Replication, DNA polymerase goes through the DNA to make sure there are no errors • This prevents mistakes in the DNA sequences • Mutations- Change in the sequence of the DNA

RNA & Protein synthesis

DNA vs. RNA • DNA can be compared to the “master plan” of a builder • It stays safely in the nucleus (office) • Double stranded • Contains thymine • Contains deoxyribose • RNA can be compared to the “disposable copies/blueprints” • It goes to the proteinbuilding ribosome site in the cytoplasm (job site) • RNA is made from the DNA • Single stranded • Contains uracil • Contains ribose

How are genes expressed? • Genes are coded DNA instructions that control the production of proteins within the cell. • The 1 st step in decoding the DNA is to copy part of the nucleotide sequence into RNA • RNA = ribonucleic acid • The RNA’s make proteins

What is RNA? • RNA consists of a long chain of nucleotides (like DNA) – A sugar called ribose – A phosphate group – A nitrogenous base Adenine (A) Uracil (U) … not Thymine (T) Cytosine (C) Guanine (G) • RNA is single-stranded

RNA Nucleotide Phosphate Group Nitrogenous Base Ribose Sugar

RNA Structure ate Phosph Group Nitrogenous Base Ribose Sugar

Types of RNA • RNA’s assemble amino acids into proteins • There are 3 types of RNA – Messenger RNA (m. RNA) – Ribosomal RNA (r. RNA) – Transfer RNA (t. RNA)

Messenger RNA • Copies instructions in genes • Serves as a “messenger” from the DNA to the cell

Ribosomal RNA • Ribosomes make proteins • Ribosomes are made of proteins and contain their own RNA

Transfer RNA • Transfer amino acids to ribosomes based on specified codes in m. RNA

How is RNA made? • RNA is made by transcription: DNA to RNA • Transcription uses an enzyme RNA polymerase • During transcription, RNA polymerase binds to DNA and separates the DNA strands, RNA polymerase then uses one strand of DNA as a template (stencil) from which nucleotides are assembled into a strand of RNA – For example: DNA: RNA: ACTGTGGACCT UGACACCUGGA http: //www. johnkyrk. com/DNAtranscription. html TRANSCRIPTION

The Genetic Code • Remember: Proteins are chains of amino acids called polypeptides • The order of the amino acids determines the properties of the protein • The instructions for making different amino acids are in the m. RNA = the genetic code • The genetic codes is read 3 letters at a time, so each “word” is 3 bases long = codon

How are proteins made? • Proteins are made by translation: RNA to protein • Translation occurs in the cytoplasm • During translation, the cell uses information from m. RNA to make proteins. • m. RNA instructs amino acids on t. RNA to join together in the ribosome containing r. RNA http: //www. johnkyrk. com/DNAtranslation. html

What is t. RNA again? • Transfer RNA brings amino acids to the ribosome. • One end of the t. RNA has an amino acid, the other end has 3 bases called the anticodon, that are complementary to one of the m. RNA codons Amino acid Anticodon

Codons • Three consecutive nucleotides that specify a single amino acid that is to be added to the polypeptide – For example: RNA: Codons: UCGCACGGU UCG-CAC-GGU Amino acids: Serine-Histidine-Glycine There are 64 possible codon combinations, but there are only 20 amino acids! *More than one codon can code for an amino acid

When does it begin or end? • There is 1 “start” codon that initiates protein synthesis if it is the 1 st codon (otherwise it codes for an amino acid) – AUG • There are 3 “stop” codons that do not code for any amino acid and signify the end of a polypeptide. – UAA – UAG – UGA

Let’s try to make a protein! • • • DNA: CCAGATAGGTTT m. RNA: GGUCUAUCCAAA codons: GGU-CUA-UCC-AAA t. RNA: CCA-GAU-AGG-UUU Amino acids for the m. RNA codons on t. RNA’s: Glycine-Leucine-Serine-Lysine