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examBoard: Pearson Edexcel
examType: IGCSE
lessonTitle: mRNA, tRNA and Ribosomes
DNA contains the genetic instructions for making proteins, but it doesn't do this job directly. Instead, DNA works with several key molecules to turn genetic information into functional proteins through a process called protein synthesis. The main players in this process are messenger RNA (mRNA), transfer RNA (tRNA) and ribosomes.
Key Definitions:
The central dogma of molecular biology explains the flow of genetic information: DNA → RNA → Protein. First, DNA is transcribed to make mRNA. Then, mRNA is translated to make proteins. This two-step process ensures that genetic information is accurately passed from DNA to proteins.
DNA can't directly make proteins because: 1) DNA is too valuable to risk damage during protein synthesis, 2) DNA stays in the nucleus while protein synthesis happens in the cytoplasm and 3) using RNA as an intermediate allows for more control over which proteins are made and when.
Messenger RNA (mRNA) is a single-stranded molecule that carries genetic information from DNA in the nucleus to the ribosomes in the cytoplasm. It serves as a template for protein synthesis.
mRNA is a long, single-stranded molecule made up of nucleotides. Each nucleotide contains a ribose sugar, a phosphate group and one of four nitrogenous bases: adenine (A), uracil (U), guanine (G), or cytosine (C). Unlike DNA, RNA uses uracil instead of thymine.
An mRNA molecule has several important regions:
A single mRNA molecule can be used to make many copies of the same protein. This amplification is important because it allows cells to quickly produce large amounts of a needed protein without having to make multiple copies of the gene.
Transcription is the process of creating an mRNA copy of a gene from DNA. It occurs in the nucleus of eukaryotic cells.
The enzyme RNA polymerase binds to the promoter region of the DNA. This marks where transcription should begin.
RNA polymerase moves along the DNA, unzipping the double helix and adding complementary RNA nucleotides to create the mRNA strand.
When RNA polymerase reaches a termination signal, it releases the newly formed mRNA and detaches from the DNA.
After transcription, the mRNA is processed (in eukaryotes) and then moves from the nucleus to the cytoplasm where it will be translated into a protein.
Transfer RNA (tRNA) is a small RNA molecule that brings amino acids to the ribosome during protein synthesis. Each tRNA molecule carries a specific amino acid and has a specific anticodon that matches a codon on the mRNA.
tRNA has a distinctive cloverleaf structure when drawn in 2D, but folds into an L-shape in 3D. Key features include:
tRNA acts as an adapter molecule. It reads the genetic code on mRNA and brings the correct amino acid to the growing protein chain. This is possible because each tRNA has a specific anticodon that pairs with a specific codon on mRNA and each tRNA carries a specific amino acid.
The genetic code is the set of rules that determines how the nucleotide sequence of mRNA is translated into the amino acid sequence of a protein. It's based on codons - sequences of three nucleotides that specify a particular amino acid or signal the start or end of translation.
Ribosomes are complex molecular machines that read the genetic code on mRNA and build proteins by linking amino acids together in the correct order.
Ribosomes are made up of ribosomal RNA (rRNA) and proteins. They consist of two subunits:
Ribosomes have three important sites:
Translation is the process of building a protein using the genetic information in mRNA. It occurs on ribosomes in the cytoplasm.
The small ribosomal subunit binds to mRNA at the start codon (AUG). The first tRNA (carrying methionine) attaches to the start codon. The large ribosomal subunit joins to form the complete ribosome.
The ribosome moves along the mRNA, reading each codon. Matching tRNAs bring amino acids, which are linked together to form a growing polypeptide chain.
When the ribosome reaches a stop codon (UAA, UAG, or UGA), translation stops. The completed protein is released and the ribosome dissociates from the mRNA.
Sickle cell anaemia is a genetic disorder caused by a single nucleotide change in the gene for haemoglobin. This change results in the substitution of valine for glutamic acid at position 6 in the beta-globin chain. This small change in the protein causes red blood cells to become sickle-shaped when oxygen levels are low, leading to various health problems. This case demonstrates how a tiny change in DNA can affect mRNA, which then affects the protein produced, ultimately causing disease.
Accuracy in protein synthesis is crucial. Errors can lead to non-functional proteins or proteins with altered functions, which can cause diseases. Several mechanisms ensure accuracy:
Protein synthesis is incredibly efficient. A typical eukaryotic cell can make about 10,000 different proteins. Ribosomes can add 2-5 amino acids per second to a growing protein chain. A single mRNA molecule can be translated multiple times, producing many copies of the same protein.
DNA does not directly make proteins - it needs mRNA as an intermediate. RNA is not just a copy of DNA - it contains uracil instead of thymine and is single-stranded. The genetic code is not a direct one-to-one mapping - most amino acids can be coded for by multiple codons (this is called the degeneracy of the genetic code).
Protein synthesis is a complex but elegant process that converts genetic information in DNA into functional proteins. mRNA carries the genetic message from DNA, tRNA brings amino acids to the ribosome and ribosomes coordinate the assembly of proteins according to the genetic code. This process is essential for life and demonstrates the remarkable molecular machinery that operates within our cells.
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