DNA Structrue & Function

Nucleic Acids Nucleic acids consist of 3 basic subunits: sugars (make up the backbone – sides of the molecule) phosphates nitrogenous bases One sugar, one phosphate, and one base make up a nucleotide Shape is known as a double helix or twisted ladder (double stranded molecule) DNA – deoxyribonucleic acid RNA – ribonucleic acid

DNA a polynucleotide (made of many nucleotides) contains the sugar deoxyribose contains the bases: adenine (purine bases) thymine (pyrimidine bases) guanine (2 rings) cytosine (1 ring) bases are held together by weak hydrogen bonds Watson & Crick – described the structure of DNA Base-pairing rules: A-T, G-C (Chargaff’s rules)

RNA a polynucleotide (made of many nucleotides) contains the sugar ribose contains the bases: adenine (purines) uracil (pyrimidines) guanine cytosine Base pairing rules: A-U, G-C RNA is single stranded Types of RNA: mRNA - messenger tRNA - transfer rRNA - ribosomal

DNA Replication process where DNA makes an exact copy of itself to give to a new cell Part of DNA unwinds and unzips – H bonds in the unwound portion break and the two strands separate, exposing 2 rows of nitrogenous bases Free DNA nucleotides (present in the nucleus) pair with exposed bases on both strands of DNA by weak hydrogen bonds following base-pairing rules This process continues throughout the molecule. The final product is 2 new DNA molecules identical to the original DNA, containing the same information 4) The 2 new DNA molecules rewind

Transcription process of mRNA synthesis Part of DNA unwinds and unzips, then the 2 strands separate (one gene in length) gene- segment of DNA that codes for a protein Free RNA nucleotides (from the cytoplasm) come in to the nucleus and pair up along one of the DNA strands and form weak H-bonds with complementary bases. This forms a strand of mRNA. This continues until the entire gene is transcribed Weak H bonds between mRNA and DNA break and mRNA molecule moves away from the DNA and out into the cytoplasm, then the two DNA strands rejoin and recoil

Translation Process of protein synthesis -cells make proteins by “reading” codons (3-base units) on mRNA molecule 1) mRNA leaves the nucleus through pores in the nuclear membrane, then to cytoplasm to ribosomes & attaches itself to a ribosome 2) tRNA in the cytoplasm attaches itself to an amino acid (AA) on one end; the opposite end contains a triplet of exposed bases known as an anticodon; the kind of anticodon determines the AA the tRNA will bond with 3) Anticodons on tRNA match up and form H-bonds with complementary codons on mRNA (beginning with a start codon – AUG) (therefore, sequence of mRNA codons directs AA sequence) ****Remember a sequence of DNA bases originally specified the mRNA sequence

Translation 4) AA’s bond together & begin to form a protein 5) tRNA’s attached to bonded AA’s break away from the AA’s (now attached to mRNA) 6) Ribosome moves down mRNA to next codon until a stop codon (does not code for any AA, includes UAA, UAG, UGA) is reached, forming a complete protein ***All proteins are synthesized in this manner. Many are synthesized simultaneously on different ribosomes in the cell. A different mRNA carries the code for each kind of protein.