DNA It’s in our Genes!

DNA-What is it? DNA stands for deoxyribonucleic acid It is a nucleic acid that contains our genetic/hereditary information (located in our nucleus) DNA’s main role is the long-term storage of information – Similar to blueprints or a recipe because it contains instructions needed to construct cells – The DNA segments that contain the genetic information are called genes

DNA’s makeup DNA consists of two long polymers of simple units called nucleotides DNA polymers can be enormous molecules containing millions of nucleotides – Nucleotides are made up of a 5 carbon sugar (in this case deoxyribose), a phosphate group and one of four nitrogenous bases (Adenine, Cytosine, Thymine and Guanine)

DNA’s structure In living organisms, DNA does not usually exist as a single molecule, but instead as a tightly-associated pair of molecules – These two long strands entwine like vines, in the shape of a double helix (or a twisted ladder) The nucleotide repeats containing both the segment of the backbone of the molecule, which holds the chain together, and a base, which interacts with the other DNA strand in the helix.

The Rest of the Story The backbone is made up of deoxyribose (the sugar) and phosphate groups of each nucleotide The bases form the rungs of the ladder and are connected by hydrogen bonds – H bonds are very weak The nucleotides can be joined together in any order

The Bases Purines – Bases adenine and guanine – Double ring structure Pyrimidines – Bases cytosine and thymine – Single ring structures

Chargaff This is a rule/principle that applies to DNA’s nucleotides The percentages of G & C and A & T are almost equal in any sample of DNA For example: in humans we are made up of about 30.9 % A and 29.4 % T (almost equal!) and about 19.9 % G with 19.8% C

X-Ray Evidence Rosalind Franklin began to study DNA using a technique called X-ray diffraction Her work did not reveal the structure of DNA but did show that there are two strands which are twisted around each other

The Double Helix Francis Crick and James Watson were trying to understand the structure of DNA at the same time Franklin was They built 3-D models in an attempt to figure it out Using Franklin’s pictures they were about to construct the model of the double helix

Complementary Pairing Also called base pairing The following bases ALWAYS pair up with one another in DNA: – A with T – G with C Each pair forms one of the rungs of the ladder

The Code DNA molecules carry the code for all the genes of an organism Genes are pieces of the DNA molecule that code for specific proteins The process of making genes into proteins is called protein synthesis-which occurs OUTSIDE of the nucleus on the ribosome

Replication Before protein synthesis occurs DNA has to replicate DNA must replicate itself and does so in the following steps: – The double strands of the DNA unzip to single strands – New DNA strands are assembled from the free- floating nucleotides in the cell’s nucleus – An enzyme known as DNA polymerase collects the bases and matches them to their complement along the single strand of DNA – When its complete, there will be two new, identical DNA double helixes

BREAK! WORK ON DNA WORKSHEET! YOU HAVE 15 MINUTES TO COMPLETE IT!

Steps of Protein Synthesis The DNA code of the gene segment must be copied in the nucleus of the cell The code must be carried from the nucleus into the cytoplasm and then to the ribosome The protein is then assembled from the code and released – These steps are carried about by RNA (Ribonucleic acid)

Carrying the Code Out RNA is a molecule that is used to translate the code from a DNA molecule into a protein It is very similar to DNA except: it is single stranded, it’s sugar is ribose and instead of thymine as a base, it uses Uracil (so A pairs with U in RNA only!) There are three types of RNA: messenger, ribosomal and transfer (ALL are involved in protein synthesis)

RNA Types

Transcription Step one of protein synthesis is the manufacturing of messenger RNA (mRNA) This making of the mRNA is called transcription – Transcription begins when a region of the DNA unwinds and separates (this separated segment is a gene) – This unwound segment serves as a template for the soon to be mRNA strand

mRNA The mRNA strand is assembled from individual RNA nucleotides that are present in the nucleus RNA polymerase (an enzyme) picks up these nucleotides and matches them to their DNA complement from the template that has just been unzipped At this point, the mRNA separates and leaves the nucleus-moving into the cytoplasm and settling on a ribosome-this is where translation begins

Translation Ribosomes are made up of ribosomal RNA (rRNA) On these ribosomes mRNA is decoded (translated) and a corresponding polypeptide (amino acids) is formed When we “decode” a chain of nucleotides we “read” it in a three base code called a codon – For example: our mRNA sequence could be AUGACAGAUUAG – The corresponding codon would be AUG ACA GAU UAG

Codons The three nucleotide codon has the specific function of corresponding to a particular amino acid How does this work? – The mRNA is bound to the surface of the ribosome at the first nucleotide segment (called a start codon) – The cytoplasm in which the rest floats contains amino acids and a third kind of RNA, transfer RNA (tRNA)

tRNA This molecule contains a three part nucleotide segment called an anticodon (this is the exact match of one mRNA codon) The anticodon corresponds exactly to one of the 20 kinds of amino acids Once the tRNA binds the amino acid it travels to the ribosome surface and there the three tRNA bases pair with their three complementary mRNA bases

Finishing Up Translation The amino acid that is bound to the tRNA is then added to the growing polypeptide chain at the surface of the ribosome The ribosome facilitates this process by moving along the mRNA chain until it reaches a stop codon (a three nucleotide segment that tells the ribosome that the translation is complete) The ribosome then releases the newly- formed polypeptide chain which moves out into the cell as a fully functioning protein

Proteins There are many proteins within every cell They provide structure but do not act as energy sources Proteins make up enzymes Enzymes help carry out reactions in the cell – In addition, they transport other molecules and are part of our bodies’ defense against diseases

Activity Look at the following mRNA chain AUG ACA GAU UAG 1. How many codons does it contain? 2. AUG stands for which nucleotide bases? 3. If you hadn’t been told, how could you determine if this was a DNA or a RNA segment? 4. AUG is a common start codon and also codes for the amino acid methionine. Which codon above is the stop codon? 5. If this strand was complete, how many amino acids would the protein contain?

Review-You should now be able to answer the following questions *Protein synthesis begins with the manufacturing of a molecule of? mRNA rRNA tRNA Nucelotide *What are ribosomes made up of? mRNA rRNA tRNA Protein *Proteins are made up of polypeptide chains. Polypeptide chains are composed of? mRNA rRNA tRNA Amino Acids

More Review *What does tRNA carry? The mRNA to the ribosome The nucleotide bases to the mRNA An amino acid to the ribosome An amino acid to the cytoplasm *Which of the following is the last step in protein synthesis? tRNA bonds to an amino acid in the cytoplasm The stop codon binds to the ribosome and the polypeptide is released DNA unravels to expose a gene segment mRNA bonds to tRNA

Last But Not Least Which sugars are found in DNA and RNA? What are proteins made up of? What role does DNA play in protein synthesis? Make sure you can define the following: – DNA*Codon*Translation – RNA*Amino Acid*Transcription – Gene*Enzyme*Complement – Anticodon*Nucleotide *Base – tRNA*Protein Synthesis*mRNA – rRNA*Start codon*Stop codon