DNA is a nucleic acid polymer made of deoxyribonucleotides. It stores genetic information in its base sequence. RNA is a nucleic acid polymer made of ribonucleotides, and it can carry or use genetic information inside a cell. Transcription is the enzyme-catalysed synthesis of RNA using one strand of DNA as a template.

A template strand is the DNA strand copied by complementary RNA nucleotides during transcription. For any particular gene, only one of the two DNA strands is used. The other strand is often called the coding strand, since its base sequence matches the RNA transcript except that DNA has thymine where RNA has uracil.

RNA polymerase is the enzyme that catalyses transcription. It separates the DNA strands locally, lines up RNA nucleotides against the template strand, and joins them into a sugar-phosphate backbone. As it moves along the DNA template, the new RNA strand grows nucleotide by nucleotide. When it reaches the end of the transcribed region, the RNA molecule is released and the DNA double helix reforms.

A gene is a sequence of DNA bases with a functional product, usually a polypeptide or a functional RNA. The same gene can be transcribed many times, so one stable DNA sequence can produce many RNA copies. In this way, cells turn stored genetic information into working molecules without using up the original DNA.

Hydrogen bonding

is a weak intermolecular attraction where a hydrogen atom covalently bonded to an electronegative atom is attracted to another electronegative atom. In nucleic acids, hydrogen bonds let bases pair in a specific way.

Complementary base pairing

is the specific pairing of nucleic acid bases due to matching hydrogen-bonding patterns. In transcription, cytosine pairs with guanine, guanine pairs with cytosine, thymine on the DNA template pairs with adenine on the RNA strand, and adenine on the DNA template pairs with uracil on the RNA strand. Say that last one carefully: DNA template A gives RNA U, not RNA T.

Transcription relies on these temporary hydrogen bonds. Together, they are strong enough to hold the correct RNA nucleotide in place while RNA polymerase forms the covalent backbone, but weak enough for the RNA transcript to separate from DNA once it has been made.

Here, hydrogen bonding really matters in biology: it makes information copying accurate, while still allowing the strands to separate after the process has finished.

During transcription, the DNA template is read, not rewritten. Its base sequence is the same before and after RNA polymerase moves along it. The two DNA strands separate only for a short time; complementary base pairing then restores the double-stranded structure.

That stability matters because the same DNA sequence can be transcribed again and again. If transcription changed the template, later RNA copies would pick up new errors. Over time, the cell would lose dependable access to the information it needs to make its proteins.

This is especially important in somatic cells, which are body cells that are not used directly to form gametes. Some somatic cells do not divide, so they must conserve important DNA sequences for the whole life of the cell. Genetic continuity between cell generations, and within long-lived cells, depends on DNA staying as a stable information store while RNA serves as a temporary working copy.

Gene expression is the process by which information in a gene is used to produce a functional product that affects the cell or organism. In most protein-coding genes, expression starts with transcription and then moves on to translation.

A cell doesn’t express every gene all the time. A liver cell, a neurone and a pancreatic beta cell contain broadly the same genome, but they use different sets of genes. Part of a cell’s identity depends on which genes are being transcribed and which ones remain silent.

Transcription comes first in the expression of a protein-coding gene, so it acts as a key control point. If transcription of a gene is switched off, no mRNA for that gene is made and the corresponding polypeptide cannot be produced. If transcription is switched on, mRNA becomes available for translation. Through this selective expression, cells with the same inherited DNA can become different in structure and function.

Messenger RNA (mRNA) is an RNA molecule that carries a base sequence copied from DNA to a ribosome, where it can be used for polypeptide synthesis. Translation is the synthesis of a polypeptide, with the base sequence of mRNA determining the amino acid sequence.

A polypeptide is a polymer of amino acids joined by peptide bonds. During translation, the cell switches molecular “language”: the nucleotide base sequence in mRNA is read to build an amino acid sequence in a polypeptide.

In eukaryotic cells, transcription takes place in the nucleus, then mRNA is exported to the cytoplasm for translation by ribosomes. Prokaryotic cells have no nucleus, but the same basic idea still applies: mRNA carries the information that directs the order of amino acids.

Translation needs three main components working together.

mRNA supplies the base sequence that will be read. It binds to the small subunit of a ribosome, so the base sequence sits in the right position for decoding.

A ribosome is a ribonucleoprotein complex that catalyses polypeptide synthesis by holding mRNA and tRNA molecules in position. It has a small subunit that binds mRNA and a large subunit that binds tRNA molecules. During translation, two tRNAs can bind to the large subunit at the same time, so the ribosome can keep the growing chain and the next amino acid in the same catalytic space.

Transfer RNA (tRNA) is an RNA molecule that carries a specific amino acid and has a base triplet that pairs with mRNA. One end of a tRNA carries an amino acid; another part of the molecule contains the bases that recognize the mRNA sequence. The tRNA must be loaded with the correct amino acid, because the ribosome checks base pairing rather than the chemical identity of the amino acid.

Put simply: mRNA is the message, tRNA is the adaptor, and the ribosome is the workbench and catalyst. Protein diversity in a cell starts here, because different mRNA sequences produce different amino acid sequences.

A codon is a three-base sequence in mRNA that specifies an amino acid or a stop signal during translation. An anticodon is a three-base sequence in tRNA that is complementary to an mRNA codon.

During translation, a tRNA anticodon pairs with an mRNA codon by complementary base pairing. In RNA-RNA pairing, adenine pairs with uracil, while cytosine pairs with guanine. If the anticodon does not match the codon closely enough, the ribosome does not hold the tRNA in the correct way.

Hydrogen bonding does another very precise job here. It lets a tRNA check whether it belongs at the next codon, then detach later as the ribosome moves along. Transcription and translation both use the same basic chemical principle: complementary bases can recognize one another reversibly.

The genetic code is the rule set that links mRNA codons to amino acids, or to stop signals, during translation.

RNA has four possible bases: adenine, uracil, cytosine and guanine. If the code used one base at a time, it could give only $4$ meanings. Two bases would give $16$. Cells have to code for $20$ amino acids, so the code needs triplets. With three bases in each codon, $64$ codons are possible, which is enough for all amino acids plus stop signals.

Most codons specify amino acids. A small number are stop signals; they do not code for an amino acid, but they help bring translation to an end.

Degeneracy is the feature of the genetic code where more than one codon can specify the same amino acid. Because of this, some base substitutions leave a polypeptide unchanged: the new codon may still code for the same amino acid.

Universality is the feature of the genetic code where almost all organisms use the same codon meanings. There are minor exceptions, but the general pattern is shared across life. This universality provides evidence for common ancestry and helps explain why genes can sometimes be expressed in a different species.

A genetic code table uses mRNA codons. Not DNA triplets, and not tRNA anticodons. Check that first before you do anything else. Standard mRNA codon table. Cells show mRNA codon → amino acid; Stop is not an amino acid.

To work out an amino acid sequence from mRNA, split the mRNA sequence into triplets from the starting point you’re given. Then read the codons in order and use the table to find the amino acid or stop signal. For example, if an mRNA sequence begins AUG, the table gives methionine, and AUG also acts as a start codon in normal translation.

If the question gives you a DNA template strand and asks for mRNA, apply complementary base pairing: DNA template A gives RNA U, DNA T gives RNA A, DNA C gives RNA G, and DNA G gives RNA C. If it gives you the coding strand of DNA instead, the mRNA sequence matches it except that U replaces T. This is where students often lose marks: they read the wrong molecule.

When you deduce a polypeptide, stop if a stop codon appears in the reading frame. A stop codon is not an amino acid, so don’t include it in the amino acid sequence.

Elongation is the repeated stage of translation where the polypeptide lengthens, one amino acid at a time. In each cycle, the ribosome moves along the mRNA by one codon, so the message is read in groups of three bases.

A charged tRNA carrying the next amino acid binds to the ribosome through codon-anticodon pairing. The ribosome then catalyses a peptide bond between the amino acid on this newly arrived tRNA and the growing polypeptide chain. The chain is passed onto the tRNA in the next position.

Once the peptide bond has formed, the ribosome shifts one codon along the mRNA. The tRNA now carrying the growing polypeptide moves into the position that holds the chain ready for the next cycle. The empty tRNA leaves the ribosome, and it can be reused after being loaded with another amino acid.

Keep the focus on elongation here: codon recognition, peptide bond formation, and ribosome movement. This step-by-step movement depends on direction, because the ribosome has to read the mRNA in one consistent direction to keep the amino acids in the correct order.

A mutation is a change in the base sequence of genetic material. A point mutation is a mutation affecting a single nucleotide position in DNA.

After transcription, a point mutation can change an mRNA codon. If the new codon codes for a different amino acid, translation makes a polypeptide with a changed primary structure. Sometimes one amino acid change does very little. In other cases, it can affect folding, bonding, solubility or the active site of a protein.

A clear example is a point mutation in the beta-globin gene. A DNA base substitution changes a codon, so the mRNA codon specifies valine instead of glutamic acid in the beta-globin polypeptide. That changes the surface properties of haemoglobin. Under low oxygen conditions, the altered haemoglobin molecules can stick together and form fibres, which distort red blood cells into a sickle shape.

This example connects protein synthesis with genetic continuity. If this kind of mutation is present in a germ-line cell that forms a gamete, it can pass to the next generation. The inherited DNA sequence then affects the mRNA sequence, the amino acid sequence, protein structure and finally the phenotype.

Directionality is the property of a nucleic acid strand having chemically different ends that determine the direction in which enzymes work. DNA and RNA strands have a $5^{\prime}$ end and a $3^{\prime}$ end because the sugar-phosphate backbone is asymmetric.

During $5^{\prime}$ to $3^{\prime}$ transcription, RNA polymerase builds the RNA strand by adding each incoming RNA nucleotide to the $3^{\prime}$ end of the growing RNA. The RNA transcript therefore grows in the $5^{\prime}$ to $3^{\prime}$ direction, while the DNA template is read the other way.

In $5^{\prime}$ to $3^{\prime}$ translation, the ribosome travels along the mRNA from its $5^{\prime}$ end towards its $3^{\prime}$ end. Codons are read in that order, so direction affects the amino acid sequence. Read the same bases in the opposite direction, and they would not produce the same polypeptide.

This answers the linking question about biological mechanisms relying on directionality. Protein synthesis has direction at both stages: RNA is made $5^{\prime}$ to $3^{\prime}$, and mRNA is translated $5^{\prime}$ to $3^{\prime}$. Directionality stops the information from being read ambiguously.

A promoter is a DNA sequence near a gene where proteins bind to start transcription. It is not translated into a polypeptide; it has a regulatory role.

Transcription starts when RNA polymerase is recruited to the promoter. Often, transcription factors help decide whether RNA polymerase can bind and begin transcription. These regulatory proteins bind specific DNA sequences. Some make transcription more likely; others reduce it or block it.

Cells use this mechanism to switch genes on and off at the transcription stage. The gene itself does not have to change for gene expression to change. Instead, the cell can alter which transcription factors are present or active, which then affects which promoters are used.

A non-coding sequence is a DNA base sequence that is not translated into a polypeptide. That doesn’t make it useless. Some non-coding sequences play essential structural, regulatory or RNA-producing roles.

The examples you need are quite specific:

• Regulators of gene expression are non-coding DNA sequences that influence whether genes are transcribed, such as promoters and other regulatory binding regions.
• Introns are non-coding sequences within eukaryotic genes that are transcribed into pre-mRNA but removed before translation.
• Telomeres are repetitive non-coding DNA sequences at the ends of eukaryotic chromosomes that help protect chromosome ends.
• Genes for rRNA and tRNA are DNA sequences whose products are functional RNAs, not polypeptides.

So a genome is not just a set of protein recipes. It also contains switches, spacers, chromosome-end protection and genes for RNA molecules that help make proteins.

Post-transcriptional modification is the processing of an RNA transcript after transcription and before it is used in translation. In eukaryotic cells, this happens before mature mRNA leaves the nucleus.

A newly transcribed eukaryotic RNA molecule from a protein-coding gene is often called pre-mRNA. For this topic, it is modified in three main ways.

First, a $5^\prime$ cap is added to the $5^\prime$ end. The $5^\prime$ cap is a modified nucleotide structure attached to the $5^\prime$ end of eukaryotic mRNA that helps protect the transcript and supports later translation.

Second, a polyA tail is added to the $3^\prime$ end. The polyA tail is a sequence of many adenine nucleotides added to the $3^\prime$ end of eukaryotic mRNA that helps stabilize the transcript.

Third, introns are removed and exons are joined. An exon is a sequence in a eukaryotic gene that remains in mature mRNA after splicing and can contribute to the translated sequence. Splicing is the RNA-processing step in which introns are removed and exons are joined to form mature mRNA.

The cap and tail protect mRNA from rapid breakdown. Splicing then creates a continuous coding sequence for translation. Because transcription happens in the nucleus and translation happens in the cytoplasm, eukaryotic cells have the time and space to process RNA before ribosomes read it.

Alternative splicing is a form of RNA processing where different combinations of exons from the same pre-mRNA are joined, producing different mature mRNA molecules.

The key idea is straightforward: one gene can give rise to more than one polypeptide. The same DNA sequence is transcribed, but the mature mRNA changes depending on which exons are kept. Those different mature mRNAs are then translated into related polypeptides with different amino acid sequences.

This increases protein diversity without needing a separate gene for every polypeptide variant. It also helps answer the linking question about how protein diversity contributes to cell function: by changing RNA processing, cells can produce protein variants suited to different tissues, developmental stages or conditions.

Translation initiation sets the reading frame and puts the first tRNAs in the right positions. Start in the wrong place, and every codon after that is read incorrectly.

The small ribosomal subunit attaches to the 5′ terminal region of the mRNA. An initiator tRNA carrying methionine pairs with the start codon, usually AUG, as the small subunit moves along the mRNA to locate it. The initiator tRNA has an anticodon complementary to AUG.

Once the start codon is recognized, the large ribosomal subunit attaches. A second tRNA can then bind at the next codon, and elongation begins.

The large ribosomal subunit has three tRNA binding sites. The A site is the aminoacyl site where the next tRNA carrying an amino acid enters. The P site is the peptidyl site where the tRNA holding the growing polypeptide is positioned. The E site is the exit site where a tRNA leaves after its amino acid has been transferred.

During elongation, tRNAs move through these sites in order: A, then P, then E. That ordered movement is another example of directionality in protein synthesis.

Many polypeptides don’t work straight after translation. Post-translational modification is the chemical or structural processing of a polypeptide after translation to produce its functional form.

A protein may need amino acids removed, folding into a stable three-dimensional shape, disulfide bonds formed, chemical groups or carbohydrate chains added, or assembly with other polypeptides or non-polypeptide components. The exact modification depends on the protein.

Insulin is the required example. Pre-proinsulin is the initial inactive polypeptide translated from insulin mRNA. After its signal sequence is removed, it becomes proinsulin, an inactive precursor. Proinsulin then folds and forms disulfide bonds. It is cut again to remove a connecting segment, leaving the A chain and B chain held together by disulfide bonds. The final product is functional insulin.

The key point is the two-stage conversion: pre-proinsulin to proinsulin, then proinsulin to insulin. Translation provides the amino acid chain; modification produces the active hormone.

A proteome is the full set of proteins present in a cell, tissue or organism at a particular time. Cells do not maintain a functional proteome by making proteins alone; they also remove proteins that are damaged, misfolded or no longer needed.

A proteasome is a protein complex that breaks selected proteins into short peptides, so their amino acids can be recycled. Proteins marked for destruction are recognized, unfolded and fed into the proteasome, where protease active sites break peptide bonds.

The short peptides produced are broken down further into amino acids in the cytoplasm. Those amino acids can then be used to synthesize new proteins.

This constant turnover isn’t wasteful. It acts as quality control. Cells need continual protein breakdown and synthesis to keep the proteome matched to current needs. Protein diversity helps cells function, but cells have to manage, refresh and repair that diversity throughout their life.