Year 12 Biology Macromolecules Unit Chapter 1: M1-4

Introduction Macromolecules are all the large molecules which are necessary for life. This includes Nucleic Acids, Proteins, Polysaccharides and Lipids. All of the molecules are composed of smaller parts called Monomers. We’re starting with DNA, which is made up of monomers called Nucleotides.

M1: DNA DNA – Abbreviation for Deoxyribonucleic Acid Contains all the information required for cell replication. Is composed of nucleotides made of sugar, phosphorous and bases that are similar to chemical puzzle pieces. There are only 4 of these bases; A, T, C & G. They are ordered like a code, and the code contains all the information that our cells need to function.

DNA Continued DNA is our genetic make-up, it is the code in the DNA which makes us who we are. It directs the synthesis (creation) of proteins. It enables cells to replicate themselves which is necessary for our growth and recovery from injuries. The shape it is formed into is called a ‘double helix’ because it has two strands which twist around each other.

M2: The Chromosome Chromosomes are the larger structures which keep our DNA organised. Normally our DNA is tangled, but during reproduction the DNA needs to be neatly packaged – this gives us the chromosome.

Chromosomes & Genes The coded information that we actually use in our DNA is organised into useful segments called genes. Most of these genes contain code specifically used for synthesising proteins. It is the shape and purpose of these proteins that produce the characteristics that make up who we are. A chromosome may contain hundreds or thousands of genes. The location of a gene on a chromosome is called it’s locus. Different species of animals have different numbers of chromosomes, humans have 46.

Chromosomes and Genes Cont. Humans have approximately 25,000 genes spread over the 46 chromosomes. Each gene has a specific location, and is only found on that specific chromosome. Each gene is unique and serves a different purpose to every other gene.

M3: Genes Genes are specific sections of DNA which code for specific polypeptide chains. Many proteins have more than one part and so have to be put together from smaller pieces. In this case, a piece which is used to make a whole protein is called a polypeptide chain.

DNA, Genes and RNA In eukaryotic cells (us) the DNA is stored safely in the nucleus of the cell. In order to keep the DNA safe, other cellular processes occur outside the nucleus. This means that the cell cannot use the genes directly from the DNA. RNA has different forms and fills in all the intermediate steps. mRNA, or ‘messenger RNA’, is like a copying system, which records the information from a gene, and carries it out to the rest of the cell. This process is called transcription, from the word ‘transcribe’ which is a very old word for ‘copy’.

RNA and its Purpose The other two types of RNA are tRNA, or ‘transfer RNA’, and rRNA, or ‘ribosomal RNA’. rRNA makes up most of the RNA in the cells and is part of the Ribosomes which work with the other forms of RNA to construct proteins. tRNA carries amino acids which are the smallest building blocks in the process of making proteins. The Ribosomes connect the tRNA to the mRNA so that the code mimics the original DNA. The amino acids that the tRNA carries are then forced to line up in the sequence needed to form polypeptide chains which form proteins.

Bases, Codons & Amino Acids Amino acids are organic molecules. They are the monomers for proteins. In total there are 20 known amino acids. Each one is coded for by a set of 3 bases. These 3 set codes are called codons. There are also start and stop codons to tell the Ribosome where to start, and where to stop.

RNA is Tricky. You may have noticed in the previous picture that there is no ‘T’ in the RNA. DNA is coded with bases A, T, C & G, but the cell will only use Thymine (T) in real DNA. This means that in RNA the T is replaced with a U (Uracil). For Reference: A = Adenine, T = Thymine, C = Cytosine, G = Guanine, U = Uracil (RNA only).

M4: Flow of Information (Transcription-Translation) In the cell information flows from the DNA, to the final product which is typically a protein. There are two processes involved in the synthesis of proteins in the cell. The first part is Transcription, where the code from the DNA is transcribed (copied) into mRNA. The second part is Translation, where the information coded on the mRNA is translated into a sequence of amino acids which form a polypeptide chain.

Recap: The molecules involved in this process are: DNA – double helix, has all the needed information, four bases: A,T,C,G. mRNA (Eukaryotic cells) – single stranded, is a copy of DNA, is only the length of one gene, has Uracil instead of Thymine. tRNA – single strands twisted into a clover shape, has an anticodon to match to the mRNA’s codon, carries amino acids. Protein – LARGE molecules (in comparison), made of amino acids formed into polypeptide chains. The purpose of the protein is determined by its shape.

Transcription Moving on…. Transcription occurs only in the nucleus. The DNA acts as a master copy for creating mRNA. As previously mentioned, protein synthesis happens in the cytoplasm outside the nucleus. This is why we need mRNA and transcription. The first step in this process is to ‘unzip’ the DNA so that the bases are exposed.

Details of Transcription The two strands in DNA are connected by the bonding of the matching bases (A,T,C,G). These bonds are weak, but still need to be broken apart before mRNA can be made. An enzyme called helicase first runs down the section of the DNA which contains the desired gene. This enzyme splits the DNA apart, exposing the bases. Free floating RNA nucleotides (bases) will then join up to the exposed bases, making a copy of the DNA’s code. When this starts to happen an enzyme called RNA polymerase follows along behind, connecting the newly attached free bases to each other in a single strand.

Transcription Details Cont. Once this new strand is complete you have a finished mRNA strand which can be sent out to the cytoplasm. This mRNA strand will have the code required for the specific chain of amino acids which are needed to make the protein which the gene is designed for. When it’s finished the DNA will re-form the double helix structure.

Translation In the process, the mRNA strand which we just created is translated from DNA code, to the coded set of amino acids needed to form the protein. This process occurs in the cytoplasm and requires a ribosome to function. The ribosome converts the code on the mRNA into a polypeptide chain. This polypeptide chain is used to form a protein, which could have many different purposes. E.g. An enzyme.

Details of Translation To start the process the mRNA arrives at the Ribosome. The ribosome itself is made of a combination of RNA and protein. The Ribosome reads the start codon on the mRNA and then attracts an appropriate piece of tRNA with an attached amino acid.

Translation Details Cont. Once the first piece of tRNA has connected the ribosome reads the next codon and attracts the next tRNA/amino acid. Once this has connected, a peptide bond is made. After the bond is formed, the first piece of tRNA is released and the mRNA moves along a spot in the Ribosome. Eventually the ribosome reaches the stop codon and the polypeptide chain is released so that it can be folded into a protein.

Translation Summary Three stages of Translation: Initiation-start codon used Chain elongation- Building of amino acid sequence from the codons Termination-completion of the mRNA sequence, with the use of a stop codon.