Cellular Control (OCR A Level Biology): Flashcards

A nonsense mutation creates a premature stop codon, causing translation to stop early and the polypeptide chain to be incomplete.

• Beneficial mutations

• Harmful mutations

• Neutral mutations

A mutation that produces an altered characteristic giving the organism a selective advantage.

Example: as humans moved into cooler climates, mutations decreased melanin production; paler-skinned individuals could synthesise more vitamin D at lower light intensities, a selective advantage.

A mutation that produces an altered characteristic with a harmful effect on the organism; many genetic diseases arise this way (e.g. haemophilia, sickle cell anaemia).

Cystic fibrosis: a deletion mutation in the gene coding for the CFTR protein causes a loss of CFTR function, leading to symptoms such as lung and pancreatic problems from extremely thickened mucus.

A mutation that offers no selective advantage or disadvantage to the organism.

This can occur because:

• The mutation does not alter the polypeptide

• It alters the polypeptide only slightly, so structure/function is unchanged

• It alters the polypeptide, but the change in characteristic gives no particular advantage or disadvantage (e.g. tasting the bitter chemical in Brussels sprouts, via the TAS2R38 gene)

• It changes the amino acid that would have been coded for

• It also has a knock-on effect, changing all the triplets further along the sequence (a frameshift mutation)

• This may dramatically change the amino acid sequence and therefore the ability of the polypeptide to function

• It changes the amino acid that would have been coded for

• It has a knock-on effect, changing all the triplets further along the sequence (a frameshift mutation)

• This may dramatically change the amino acid sequence and therefore the ability of the polypeptide to function

False — only insertions and deletions cause a frameshift; a substitution changes only the single triplet in which it occurs.

False — a silent mutation does not change the amino acid sequence, because the genetic code is degenerate.

Yes.

The nucleus of every cell in the human body contains the same genes. However, not every gene is expressed in every cell, and genes are not all expressed all of the time.

To ensure the correct genes are expressed in the correct cell at the correct time.

Although every cell contains the same genes, different cells need different proteins, and requirements change over time (e.g. during development).

Mechanisms within cells that control which genes are expressed at different points in time (e.g. during development), ensuring the correct genes are expressed in the correct cell at the correct time.

• Regulation at the transcriptional level (during transcription)

• Regulation at the post-transcriptional level (after transcription)

• Regulation at the post-translational level (after translation)

During transcription.

These regulatory mechanisms control whether a gene is transcribed into mRNA.

After transcription (but before or during translation).

These regulatory mechanisms act on the RNA once it has been transcribed.

After translation.

These regulatory mechanisms act on the protein once it has been produced.

A gene that codes for a protein with a function within a cell (e.g. enzymes, membrane carriers, hormones).

For example, the F8 gene codes for Factor VIII, a protein involved in blood clotting.

A gene that codes for proteins (or various forms of RNA) that control the expression of structural genes.

• A structural gene codes for a protein with a direct function in the cell (e.g. an enzyme, membrane carrier or hormone).

• A regulatory gene codes for proteins or RNA that control the expression of structural genes.

Regulatory genes code for proteins or various forms of RNA that control the expression of structural genes.

Structural genes code for proteins that have a function within a cell, such as enzymes, membrane carriers and hormones.

True — the nucleus of every cell contains the same genes, but not every gene is expressed in every cell, and not all are expressed all of the time.

False — a regulatory gene controls the expression of structural genes; a structural gene codes for a protein with a function in the cell.

A group (cluster) of structural genes in prokaryotes that are controlled by the same promoter.

A gene that controls structural genes and their levels of protein production, sometimes controlling several at once.

An enzyme that is only synthesised when its substrate is present, e.g. lactase, which is only made when lactose is present.

At the transcriptional level — the regulatory mechanism occurs during transcription.

It breaks down the substrate lactose.

It is only synthesised when lactose is present, which prevents the bacterium from wasting energy and materials making the enzyme when it is not needed.

• Promoter (for structural genes)

• Operator

• Structural gene lacZ

• Structural gene lacY

• Structural gene lacA

It codes for the lac repressor protein.

• The operator in the lac operon

• Lactose (the effector molecule)

When the repressor is bound to the operator, RNA polymerase cannot attach to the promoter, so transcription of the structural genes does not take place.

• The regulatory gene is transcribed and translated to produce lac repressor protein

• The repressor protein binds to the operator region upstream of lacZ

• RNA polymerase is unable to bind to the promoter region

• Transcription of the structural genes does not occur, so no lactase is synthesised

• Lactose is taken up by the bacterium

• Lactose binds to the second binding site on the repressor protein, distorting its shape so it cannot bind to the operator

• RNA polymerase can now bind to the promoter and transcription takes place

• The mRNA from all three structural genes is translated, so lactase is produced and lactose is broken down for energy

When lactose binds to the repressor protein, the shape of the repressor distorts so that it can no longer bind to the operator.

For a repressible enzyme, the effector molecule binding to the repressor helps it bind to the operator, preventing transcription.

This is the opposite of the lac operon: when there is less effector, the repressor cannot bind the operator, so transcription goes ahead and the enzyme is produced.

The lactose can then be used as an energy source in the bacterial cell.

• lacZ — codes for lactase

• lacY — codes for permease (allows lactose into the cell)

• lacA — codes for transacetylase

To the left (upstream) of the lac operon on the bacterium's DNA, with its own promoter.

True

False — lactose binds to the repressor, so it cannot bind the operator and transcription goes ahead.

A protein that binds to a specific region of DNA to control the transcription of genes (i.e. to control gene expression).

They bind to the promoter region of the gene.

• They can allow (increase the rate of) transcription

• They can prevent (decrease the rate of) transcription

• They allow organisms to respond to their environment by controlling which genes are expressed

• Some hormones achieve their effect via transcription factors

Oestrogen is a lipid-soluble molecule, so it can diffuse through the plasma membrane of a cell.

The oestrogen receptor is a transcription factor.

Once oestrogen binds to it, the receptor can initiate transcription of many genes by binding to their promoter regions.

• Oestrogen is lipid-soluble, so it diffuses through the plasma membrane and moves to the nucleus

• It binds to an oestrogen receptor, which is a transcription factor

• Binding causes a change in shape of the receptor

• The receptor moves away from the protein complex it is attached to and binds to the promoter region of a target gene

• This allows RNA polymerase to bind and begin transcribing the gene

Gibberellin controls seed germination.

DELLA is a repressor protein.

It binds to the transcription factor and prevents it from binding to the promoter of the amylase gene, so no transcription can occur.

• Initially, DELLA protein is bound to the transcription factor, preventing transcription of the amylase gene

• Gibberellin binds to a gibberellin receptor and enzyme, which starts the breakdown of DELLA

• The transcription factor is no longer bound to DELLA, so it binds to the promoter of the amylase gene

• Transcription of the amylase gene begins

• Amylase is produced

In the gibberellin pathway, the breakdown of the repressor protein DELLA is necessary for the synthesis of amylase.

When a cell expresses a gene, RNA is produced by transcription.

Scientists can match this RNA to specific genes to work out which genes are being expressed in that cell, even though cells may look identical and contain the same DNA.

It is located upstream of the gene and controls the expression of that gene.

It stimulates the synthesis of the enzyme amylase, by influencing the transcription of the amylase gene.

False — oestrogen is lipid-soluble, so it diffuses through the membrane and binds to a receptor inside the cell.

True

The region of DNA upstream of a gene where transcription factors and RNA polymerase bind to control transcription.

A coding sequence of DNA (or RNA) that is translated into the amino acids forming the final polypeptide.

A non-coding sequence of DNA (or RNA) that is not translated and does not code for any amino acids.

Both exons and introns are transcribed, so the pre-mRNA contains both coding and non-coding sequences.

Modification of an RNA molecule after transcription but before translation, including splicing to produce mature mRNA.

The process during post-transcriptional modification in which introns are removed from pre-mRNA and the exons are fused together to form a continuous mature mRNA molecule.

• The introns (non-coding sequences) are removed from the pre-mRNA

• The exons (coding sequences) are fused together to form a continuous molecule

• This produces mature mRNA that is ready to be translated

Splicing ensures that only the coding sections (exons) are used to form proteins. If introns were included in the mature mRNA, the resulting protein would not be formed properly and may not function as it should.

The RNA molecule produced directly from transcription, containing both exons and introns, is called pre-mRNA (or primary mRNA).

Splicing is the process by which introns are removed and exons are joined together to form mature mRNA.

cAMP (cyclic AMP) is a molecule derived from ATP that activates protein kinases within the cell.

• PKA is an inactive precursor enzyme

• cAMP activates PKA

• Once activated, PKA can activate other proteins (e.g. other enzymes)

When muscle cells require energy, glycogen phosphorylase releases glucose from glycogen. This enzyme is activated by cAMP, which changes the shape of the enzyme to expose its active site.

After translation, polypeptides undergo modifications in the Golgi apparatus or in the cytosol.

cAMP is formed from ATP by the action of the enzyme adenyl cyclase.

False — exons are the coding sequences; introns are non-coding and are removed during splicing.

True

The basic pattern of an organism's body, including its polarity, its segmentation into distinct body parts, and the locations at which body parts such as wings, limbs and internal organs develop.

The feature of a body plan that determines where the head/tail and front/back of an organism develop.

• The polarity of the organism (position of head/tail and front/back)

• The segmentation into distinct body parts (e.g. thorax and abdomen)

• The location at which body parts such as wings, limbs and internal organs develop

At the embryo stage of development.

A family of genes that code for transcription factors and control the development of an organism's body plan during early development.

Transcription factors.

• They control which genes are expressed at a particular time and in particular cells

• Gene expression is switched on or off in a tightly controlled sequence during early development

• This ensures cells in the correct locations differentiate into the correct cell types (e.g. cells at the top of the spinal cord becoming nerve cells for brain development)

Mutations that change homeobox sequences often make organisms unable to survive, so the mutated alleles are not passed on.

This strong negative selection pressure keeps the sequences highly conserved (very similar) across different groups of organisms.

Because they control which genes function at different stages of development, regulating the overall development of the body plan.

A subset of homeobox genes found in most animals.

Into groups known as Hox clusters, with a linear order to the Hox genes in each cluster.

Hox genes determine the identity of embryonic body regions along the head-tail axis of an animal.

Homeobox genes code for transcription factors that control the development of an organism's body plan.

A family of genes known as homeobox genes.

The identity of embryonic body regions along the head-tail axis.

The linear order of the genes is directly related to the order of the body regions they affect.

True

False — Hox genes are a subset of homeobox genes found in most animals, not in plants or fungi.

Cell division that produces identical new cells for growth, cell replacement and tissue repair.

Programmed cell death.

• Enzymes digest the cell contents

• The cell breaks apart into small fragments

• Phagocytes engulf any remains

By constantly producing and destroying cells throughout early development:

• Mitosis occurs in regions where more cells are needed

• Apoptosis occurs in regions where cells need to be removed

Structures like fingers and toes first develop as a single combined unit, and are then separated later by the removal (apoptosis) of the cells in between the digits.

Destroying infected, damaged or old body cells.

They stimulate cell division.

They reduce cell division. Tumour-suppressor genes can also stimulate apoptosis.

Proto-oncogenes stimulate cell division, whereas tumour-suppressor genes reduce cell division.

They sense issues such as DNA damage or faulty spindle attachment. Progression through the cell cycle is paused at cell checkpoints if issues are detected.

Mitosis will be prevented, and apoptosis may be triggered.

• DNA damage that cannot be repaired

• Activation of genes that stimulate cell division

• Metabolic stress, e.g. ATP depletion

• Cell signalling molecules such as cytokines and hormones

• The presence of pathogens

• Chemicals in the environment

False — apoptosis is programmed (controlled) cell death.

True