Proteins and Nucleic Acids

Proteins

Proteins are molecules with C,O,H,N and in some sulfur. They are used for enzymes, hormones, cell growth and repair, and haemoglobin. They have a general structure of an amino group(NH2) on one end, with a carboxyl group at the other end. They are both bonded to a central carbon, which is bonded to a hydrogen and an R group. The R group is the residual group, which changes to form 20 different amino acids. Two amino acids bond to each other by a peptide bond, formed through a condensation reaction(OH of carboxyl with H of amino) to form a dipeptide. More than two amino acids bonded together is a polypeptide.

Primary and secondary protein structures.

Primary structures: This is the sequence and types of amino acids. This determines the secondary and tertiary structure.

Secondary structures: There are two main types: Alpha helix and beta pleated sheets. The alpha helix is held by hydrogen bonds between two amino acids in the same chain. the beta pleated sheets have hydrogen bonds between two polypeptide sheets.

Tertiary and quaternary structures

Tertiary: This is when the secondary structure further folds into 3d shapes. There are two main types of tertiary structures: Globular and fibrous. Globular structures are round, compact and mostly soluble in water, while fibrous structures are insoluble. They are important in metabolic reactions. All enzymes are globular, since the 3d shape is needed for the active site. Haemoglobin, antibodies, and antigens are also globular. Fibrous proteins have long and rope like structures. They are used for structural support like collagen, keratin and elastin. Tertiary structures are held by many bonds, which include hydrogen, ionic, disulfate, and hydrophobic interactions. Disulfate bonds are strong covalent bonds between two sulfurs(e.g. cysteine) .Usually, hydrophyllic amino acids are on the outside, while hydrophobic ones are on the inside.. Quaternary structures are more than one polypeptide chains working together as a whole.

Haemoglobin and insulin

Haemoglobin is a conjugated globular protein which has the function of carrying oxygen around the body. A conjugated protein is one with a prosthetic group. A prosthetic group is a non protein group which is permanently bound to the protein and is vital in its function. The prosthetic group in haemoglobin is the heme group, which contains iron in it. Haemoglobin is made up of two alpha helix and two beta pleated chains, each bound to a heme group. Insulin is an amino acid which has two chains held together by 3 disulfate bonds.

Collagen

Collagen provides structural support in many areas including the skin, heart, and tendons. It is made up of 3 polypeptide chains wrapped together like a rope, which means it has a quaternary structure.The chains are bonded by many hydrogen and covalent bonds to each other, providing strength, This makes collagen fibrils which link up to make collagen fibres, Collagen is made from 3 repeating amino acids. Elastin can stretch which is why it in connective tissues.Keratin is the protective layer for skin and hair.

Nucleotides

They are the monomers for nucleic acid. They are made up of 3 components, a nitrogenous base, a phosphate, and a pentose sugar. The pentose sugar can be ribose and deoxyribose. There are 5 bases, Adenine, thymine, guanine, cytosine, uracil. Uracil replaces thymine in ribonucleic acid. ATP is a nucleotide. Nitrogenous bases can be purines or pyrimidines. Purines(A,G) have 2 carbon rings, while pyrimidines(T,C,U) have only 1.

DNA

Made of nucleotides with the pentose sugar deoxyribose. The phosphate bonds to the carbon 3 of one nucleotide and the carbon 5 of the other forming a phosphodiester bond through a condensation reaction. This forms the sugar-phosphate backbone of DNA with the nitrogenous bases sticking out. In DNA, two polynucleotide chains form hydrogen bonds together through complementary nitrogenous bases. Adenine forms two hydrogen bonds with thymine, cytosine forms three hydrogen bonds with guanine.This is called complementary base pairing. This makes the molecule more stable. DNA then twists to form a double helix structure.The two chains are antiparallel to each other, so they go in opposite directions.

Semi conservative DNA replication

An enzyme called helicase unwinds and unzips DNA by breaking the hydrogen bonds between the bases. This forms to separate DNA strands. Free nucleotides attach to the nucleotides in the DNA chain through complementary base pairing with A-T, C-G. Then the nucleotides form hydrogen bonds with their complementary base. Then, DNA polymerase covalently links the nucleotides through a condensation reaction forming a phosphodiester bond. This forms a dna chain identical to the original one, and 2 dna copies are made. This is called semi conservative since every new copy has one of the tempelate copies.

The gene

Genes are a section of DNA that codes for one protein. Every 3 bases code for an amino acid. This triplet is called a codon. So, there are 64 possible codons while there are only 20 amino acids. 3 codons signal the end of the message, while the other 61 are amino acid coding. This means that one amino acid can have more than one possible code. This is advantageous, as if a mutation happens it may still code for the same protein. Genes vary in length since proteins vary in length.