JJA 1.2 RNA & Protein Synthesis 2025 PDF

Stage 2 Biology 1.2 RNA & Protein Synthesis Science Understanding A gene consists of a unique sequence of nucleotides that codes for a functional protein or RNA molecule. Protein synthesis involves transcription of a gene into messenger RNA (mRNA) and translation of mRNA into an amino acid se...

Stage 2 Biology 1.2 RNA & Protein Synthesis Science Understanding A gene consists of a unique sequence of nucleotides that codes for a functional protein or RNA molecule. Protein synthesis involves transcription of a gene into messenger RNA (mRNA) and translation of mRNA into an amino acid sequence at the ribosomes. In eukaryotic cells, transcription occurs in the nucleus. Content Checklist Distinguish between exons and introns as coding/non-coding segments of DNA found in genes in eukaryotes. Describe how both exons and introns are transcribed but only the information contained in exons is translated to form a polypeptide in eukaryotes. Describe and illustrate the role of DNA, mRNA, tRNA and rRNA in transcription and translation. Describe the relationship between DNA codons, RNA codons, anticodons and amino acids. Distinguish between coding (gene) and template strands of DNA. Recognise that DNA strands are directional and are read 5’ → 3’  Genes - recap A gene consists of a unique sequence of nucleotides that codes for a polypeptide chain (functional protein) or an RNA molecule. Genes are heritable factors that control specific characteristics and are located on specific points of the chromosome (locus). RNA  Ribonucleic acid - RNA RNA is found in all living organisms and is made up of nucleotides. RNA is critical for the expression and regulation of genes. RNA nucleotides consist of: Pentose sugar (5 carbon) called ribose. A phosphate group. Organic base (A, C, G and U - uracil). RNA is a single stranded molecule. When RNA molecules are synthesised, they will bond with their complementary base on the template strand of the DNA molecule. A=T C≡G  The Genetic Code - Codons The Genetic Code - Codons The bases along DNA and RNA are divided into triplets of bases. AGG CUA AAU These triplets are known as codons and will code for all 20 amino acids necessary for making proteins. Both DNA and RNA have codons and anticodons. Predominantly, the term codon refers to the codons along the mRNA as that is what will synthesise the protein. Some codons are known as ‘stop’ codons as they will signal that the synthesis of a protein is complete. The genetic code 43 = 64 4 amino acids in DNA. 3 positions in a codon We will revisit this code later when discussing translation Summary of codons Triplets of bases are the smallest units necessary to code for all 20 amino acids. Triplets of bases are codons and are found on mRNA molecules that transfer a version of the genetic code from the nucleus to the ribosomes (located in the cytosol). The number of nucleotides that make up the mRNA must be three times the number of amino acids in the protein. Some triplets are ‘stop’ codons to terminate the production line of the polypeptide chain. Polypeptides A polypeptide is a polymer/chain of amino acids linked together by peptide bonds. The sequence of amino acids in a polypeptide chain is determined by the genetic code encoded in DNA and transcribed into messenger RNA (mRNA). This sequence is critical because it determines the three-dimensional structure and therefore, function of the protein. Proteins play essential roles in the structure, function, and regulation of cells and organisms. When multiple polypeptide chains come together and fold into a specific structure, they form a functional protein. Polypeptides  Amino acids are the building blocks of polypeptides and proteins. Remember A polypeptide consists of a chain of amino acids linked together. Protein molecules can consist of one polypeptide chain folded into a precise shape OR more than one, linked together to form a functional protein. Types of RNA  messenger RNA - mRNA mRNA is synthesised using DNA as a template in transcription. Occurs in the nucleus of the cell. Undergoes modification to then move from the nucleus to the cytosol. mRNA is a single stranded molecule of RNA nucleotides.  Transfer RNA - tRNA tRNA is approximately 80 nucleotides long. Each tRNA molecule contains an anticodon consisting of 3 bases that are complementary to a specific mRNA codon. tRNA carries a specific amino acid at the opposite end of the anticodon. The function of tRNA is to place a specific amino acid into its correct sequence in the polypeptide being synthesised. Enzymes with a specific shape support attaching the tRNA with an amino acid.  Ribosomal RNA - rRNA Ribosomes are made up of ribosomal RNA (rRNA) and protein molecules. Ribosomes is where translation occurs in protein synthesis. The ribosomes are often attached to the ER (Rough ER). Ribosomes move along the mRNA strand translating the mRNA code into a sequence of amino acids (polypeptide molecule).  MicroRNA - miRNA Micro RNA is extremely important in regulating gene expression after transcription has occurred. It is a non-coding RNA molecule. They regulate gene expression by binding to a specific mRNA. miRNA stops translation by binding in a complementary fashion to the mRNA which deactivates it by: destroying the mRNA or blocking the translation. If this did not occur, the mRNA would continue to be translated. miRNA is not involved in transcription. MicroRNA - miRNA Exons and Introns Exons and Introns DNA can be divided into two parts, coding DNA and non-coding DNA. Exons: the coding DNA (exiting the cell & are exhibited) is the DNA that is transcribed into a molecule of messenger RNA (mRNA) and then translated into a polypeptide or protein molecule. Only 2% of human DNA are exons. Introns: non-coding DNA that is only transcribed into a pre mRNA molecule → it does not code for proteins. 98% of human DNA are introns. The non-coding DNA of genes codes for other types of RNA molecules including: - Ribosomal - rRNA - involved in translation. - Transfer - tRNA - involved in translation. - Micro - miRNA - involved in the regulation of gene expression. Genes consist of introns and exons. Therefore, the introns must be spliced out (RNA splicing) to ensure the correct sequence of amino acids are present to synthesise the protein. Exons & Introns - summary Exons Introns Exons are the coding regions of a gene. They Introns are non-coding regions of a gene. They contain the information necessary for protein do not contain the information necessary for synthesis. protein synthesis. When a gene is transcribed (the process of After transcription, the entire gene (including copying DNA into RNA), the exons are both exons and introns) is initially transcribed transcribed into mRNA (messenger RNA). into a pre-mRNA (precursor mRNA). mRNA serves as a template for protein Introns must be removed from the pre-mRNA synthesis during translation, and it is the exons before it can be translated into a functional that contain the actual genetic code for protein. This process is called RNA splicing. building proteins. 2% of Human DNA 98% of Human DNA  Protein Synthesis - Transcription  Protein synthesis The genes within our DNA is the very code for our body to function. Think of the gene as the recipe book and our body has to transcribe that code into a form it can translate (read) that code to make something. The ‘something’ is generally a protein. Protein synthesis involves transcription of a gene into messenger RNA (mRNA) and translation of mRNA into an amino acid sequence at the ribosomes.  Transcription A-U C-G Transcription occurs in the nucleus of the cell. It is the first step in gene expression. The mRNA is synthesised from a gene on the DNA. An enzyme, RNA polymerase separates the DNA strand (breaking the weak hydrogen bonds). It anneals (binds) the free RNA nucleotides in the nucleus together in the 5’ → 3’ direction as they complementary base pair along the template strand. RNA nucleotides are added to the 3’ end of the DNA template strand.  Transcription - coding and template strand The coding strand is the gene that will be expressed in translation. The template strand is what the mRNA complementary pairs with as it will be an exact copy of the coding strand due to complementary base pairing.  Transcription (cont) A-U C-G Once transcription of the gene has completed, the premature mRNA (pre-RNA) breaks away from the DNA and the DNA reforms. The pre-RNA now undergoes RNA splicing to remove the non-coding regions (introns). This is done by RNA spliceosome, a RNA protein. This is so the exons are present in the mature mRNA strand for protein synthesis. The mRNA moves through nuclear pores and into the cytoplasm of the cell. Transcription - summary Transcription occurs in the nucleus DNA → RNA DNA strands separate at the site of the gene One strand of the DNA is used as a template A-U for mRNA synthesis (base pairing rules apply C-G except U joins with A) mRNA breaks away from the DNA and travels through nuclear pores to ribosomes in the cytoplasm DNA strand rejoins  Protein Synthesis - Translation  Translation Translation is where the mRNA code is translated into a polypeptide chain to make a protein. Translation involves three stages: 1. Initiation The use of the start codon AUG 2. Chain The building of the amino acid sequence from the codons elongation 3. Termination The completion of the mRNA sequence with a stop codon: UAG, UAA or UGA  Translation (cont) Translation occurs along the ribosome in the cytoplasm of the cell. Ribosomes are generally found on the rough ER and have two tRNA binding sites. Ribosomes will read the mRNA in a 5’ → 3’ direction with the start codon AUG.  Translation (cont) tRNA molecules in the cell will move to the ribosome and bind with the mRNA to form a growing polypeptide chain. The ribosome facilitates the formation of the peptide bond between the amino acids. This process continues until a stop codon is read. This signals that the polypeptide chain is to be release from the ribosome. miRNA is critical in stopping this entire process by deactivating the mRNA.  tRNA - REMINDER Each tRNA molecule contains an anticodon consisting of 3 bases that are complementary to a specific mRNA codon. tRNA carries a specific amino acid at the opposite end of the anticodon. The function of tRNA is to place a specific amino acid into its correct sequence in the polypeptide being synthesised. Translation Translation Translation Translation - complete the code DNA anticodon mRNA codon tRNA anticodon Amino acid (template strand) transcribed AAA GCG TAT AUA (isoleucine) Remember! A-U C-G Translation - the code DNA anticodon mRNA codon tRNA anticodon Amino acid (template strand) transcribed AAA UUU AAA UUU (phenylalanine) GCG CGC GCG CGC (arginine) TAT AUA UAU AUA (isoleucine) Translation - summary Translation occurs in the cytoplasm RNA → protein mRNA attaches to ribosome. tRNA molecules bring specific amino acids to the ribosome, according to the codon on the mRNA. Complementary base triplets on tRNA are called anticodons Polypeptide chain grows as the amino acids are joined. Completed polypeptide breaks away from ribosome and folds to form a 3D protein.