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examBoard: Pearson Edexcel
examType: IGCSE
lessonTitle: Ribosomes and Protein Synthesis
Proteins are essential molecules that do most of the work in our cells and are required for the structure, function and regulation of the body's tissues and organs. But how does your body make these complex molecules? The answer lies in tiny cellular structures called ribosomes and a process known as protein synthesis.
Key Definitions:
Ribosomes are tiny but mighty! They're made of proteins and ribosomal RNA (rRNA) and consist of two subunits - a small one and a large one. These subunits join together during protein synthesis. Ribosomes can be found floating freely in the cytoplasm or attached to the rough endoplasmic reticulum.
Ribosomes have one main job: they're the protein factories of the cell. They read the genetic instructions carried by messenger RNA (mRNA) and use these instructions to string together amino acids in the correct order to form proteins. Think of them as the builders following a blueprint!
Before we dive into protein synthesis, we need to understand the genetic code and how it's used.
DNA is like an instruction manual for your cells. It contains genes, which are specific sections that code for proteins. DNA is made up of four nucleotide bases: Adenine (A), Thymine (T), Guanine (G) and Cytosine (C). These bases pair up (A with T, G with C) and form the famous double helix structure.
But DNA stays safely tucked away in the nucleus, while protein synthesis happens in the cytoplasm. So how do the instructions get from the nucleus to the ribosomes? That's where RNA comes in!
mRNA carries the genetic code from DNA in the nucleus to the ribosomes in the cytoplasm. It's like a photocopy of a specific section of DNA.
tRNA brings amino acids to the ribosome during protein synthesis. Each tRNA carries a specific amino acid and has an anticodon that matches a codon on the mRNA.
rRNA forms part of the structure of ribosomes and helps catalyse the formation of bonds between amino acids during protein synthesis.
Protein synthesis happens in two main stages: transcription and translation.
Transcription takes place in the nucleus and involves copying the genetic information from DNA to create mRNA.
The mRNA then undergoes some processing before leaving the nucleus through nuclear pores and heading to the ribosomes in the cytoplasm.
The genetic code is read in groups of three nucleotides called codons. Each codon specifies a particular amino acid or signals the start or end of protein synthesis. For example, the codon AUG codes for the amino acid methionine and also serves as the start codon for protein synthesis. There are 64 possible codons but only 20 amino acids, which means some amino acids are coded for by more than one codon. This is called the degeneracy of the genetic code.
Translation occurs at the ribosomes and involves converting the mRNA code into a chain of amino acids to form a protein.
After translation, the chain of amino acids (called a polypeptide) folds into a specific 3D shape to become a functional protein. The shape is determined by the sequence of amino acids. Some proteins undergo further modifications before they're ready to do their job in the cell. Proteins have countless functions, from enzymes that speed up chemical reactions to structural components that give cells their shape.
Errors in DNA, called mutations, can lead to changes in the amino acid sequence of proteins. This might result in proteins that don't work properly or at all. Some genetic disorders, like cystic fibrosis and sickle cell anaemia, are caused by mutations that affect protein synthesis. Understanding protein synthesis has helped scientists develop treatments for these conditions.
Many antibiotics work by targeting bacterial ribosomes and disrupting protein synthesis. For example, tetracyclines bind to the small subunit of bacterial ribosomes, preventing tRNA from attaching. This stops bacteria from making the proteins they need to survive and multiply. Because bacterial ribosomes are different from human ribosomes, these antibiotics can kill bacteria without harming human cells. However, bacteria can develop resistance to antibiotics, which is becoming a serious global health problem.
Protein synthesis is happening constantly in your cells. Your body needs to make new proteins for growth and repair, to replace old or damaged proteins and to respond to changing conditions. For example, when you exercise and build muscle, your muscle cells increase their rate of protein synthesis to make more muscle proteins.
Understanding protein synthesis has led to many medical advances, from producing insulin for diabetics to developing new cancer treatments that target specific proteins. It's also the basis for genetic engineering, where scientists can insert genes into organisms to make them produce useful proteins, like human insulin produced by bacteria.
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