Nucleic Acid
• Store, transmit, help express hereditary information • What determines a polypeptide’s amino acid sequence? – Programmed by discrete unit of inheritance called a GENE → made of DNA • Nucleic acids are polymers made of monomers called NUCLEOTIDES
• 2 types of nucleic acids – DNA (deoxyribonucleic acid) → Provides direction for its own replication → Directs RNA synthesis → Controls protein synthesis through RNA – RNA (ribonucleic acid)
• Genetic material organisms inherit from parents • Each chromosome is one long piece of DNA, with 100s or more genes • When cell divides to reproduce itself, DNA is copied and passed on • DNA contains info to program all of the cell’s activities
• DNA does not carry out all of the activities directly → Proteins do • Where does RNA fit in? • Each gene directs the synthesis of a type of RNA called messenger RNA (mRNA) DNA → RNA → protein • Gene expression – Process of RNA synthesis – Protein synthesis (which occurs in RIBOSOMES, found within all living cells, that serves as the site of biological protein synthesis) by means of RNA
• Polymers are polynucleotides • Each monomer is a nucleotide • Nucleotide is made of 3 parts:
- Nitrogenous base
- Pentose sugar
- One or more phosphate groups (In a polynucleotide there is only one)
• Nucleoside = sugar + nitrogenous base
- Nitrogenous bases – One or two rings with N atoms • Two types:
- Pyrimidines • One 6-membered ring • C (Cytosine), T (Thiamine), U (Uracil)
- Purines • One 6-membered ring fused to a 5-membered ring • A (Adenine), G (Guanine)
• In both DNA and RNA – Adenine – Guanine – Cytosine • In DNA only – Thymine • In RNA only – Uracil
- Nitrogenous base is attached to a sugar – Deoxyribose (DNA) – Ribose (RNA)
• The difference: one lacks an O atom → i.e. Deoxyribose • Carbons on the sugar are labeled with a prime (‘) – Second carbon in sugar ring is 2’
- Phosphate group is attached to the 5’ carbon of the sugar • Nucleotide monomers linked to make a polynucleotide – Dehydration reaction • Phosphodiester linkages join nucleotides – Phosphate group links the sugars of 2 nucleotides • This creates a repeating pattern of sugar-phosphate units – Phosphate-sugar backbone
• Nitrogenous bases are NOT part of the backbone • The two free ends of the polynucleotide are different – Phosphate attached to the 5’ carbon (5’ end) – -OH group on a 3’ carbon (3’ end)
• Directionality along backbone – 5’ to 3’ • Sequence of bases along DNA or mRNA polymer is unique for each gene • Genes are 100s to 1000s of nucleotides long • A gene’s meaning is encoded in the sequence of the four DNA bases • 5’-ATGCGT-3’ vs 5’-TAGCTTT-3’ • Linear order of bases specifies amino acid sequence • Amino acid sequence specifies protein’s 3-D structure, and therefore function
• Structure of DNA – 2 polynucleotides (strands) that spiral around an imaginary axis, forming a double helix – 2 backbones run in opposite 5’ to 3’ directions from one another → ANTIPARALLEL
• 2 DNA strands held together by H bonds between paired bases • Only certain bases are compatible with each other – A pairs with T, G with C
• If we know the sequence of one strand, we know the sequence of the other • 5’-AGGTCCG-3’ • 3’-TCCAGGC-5’ • Strands are complementary – Key feature making it possible to make two identical copies of DNA in a cell about to divide
• Chargaff’s rule of complementation – If you know one side of the strand, you can figure out the other – A always binds with T (in DNA) and U (in RNA) – C always binds with G • TTGCAGGTA • AACGTCCAT – Amount of C equals amount of G – Amount of T equals amount of A
• For example, if one double helix of DNA is 30% A. – How much T? • 30% – How much C? • 20% – How much G? • 20%
• RNA exist as single strands • RNA are versatile, probably occurred before DNA as a carrier of genetic info • Base-pairing in RNA can happen between – Regions of 2 RNA molecules – 2 stretches of nucleotides in the same RNA • tRNA
• In RNA – G pairs with C – A pairs with U → No T in RNA