Gene control

In an immune response, only B-lymphocytes whose receptors match the antigens present become activated. This activation happens when an antigen attaches to a receptor on a B-lymphocyte. The activated B-lymphocytes enlarge and then undergo mitosis. Numerous further mitotic divisions then take place, which increases the number of B-lymphocytes carrying receptors specific to the antigen. In time, cells formed in this sequence will differentiate into either plasma cells that secrete antibodies or memory B-cells. Fig. 3.1 gives a summary of B-lymphocyte activation and the events that follow. The formation of plasma cells and memory B-cells in this sequence depends on transcription factors.

Feb/March 2019

Interferon-alpha (IFN-α) may be manufactured as a recombinant human protein to treat certain cancers. The IFNA2 gene codes for IFN-α. One way to make recombinant IFN-α is to use genetically engineered Escherichia coli bacteria carrying recombinant plasmids. Each recombinant plasmid includes the IFNA2 gene; three lac operon regulatory sequences (promoter, operator and lacI); and an antibiotic resistance gene, AMPR. The sequences for the lacI gene and AMPR gene each have their own promoter. Therefore, these genes are always expressed in E. coli bacteria that contain this recombinant plasmid. Fig. 2.1 shows the recombinant plasmid. The promoter regions of the lacI gene and AMPR gene are not shown.

Feb/March 2023

Fig. 3.1 shows a sketch of a cross-section through an ovarian follicle.

May/June 2011

Membrane proteins that export sugars from cells have been identified as SWEETs. They are present in the cell-surface membranes of both animal and plant cells, including mammalian liver cells and rice mesophyll cells. Each SWEET is a protein with seven coiled regions that together form a pore across the membrane bilayer, as shown in Fig. 3.1.

May/June 2014

Section B. Answer any one question.

May/June 2016

The lac operon is a stretch of DNA found in the genome of the bacterium Escherichia coli. Its structural genes are fully expressed only when E. coli is exposed to a high concentration of lactose.

May/June 2018

Interpupillary distance (IPD) means the separation, in millimetres, between the centres of the pupils. Fig. 2.1 illustrates how IPD is measured. IPD is one example of a feature of human facial structure that varies. Fig. 2.2 shows the distribution of IPD values in a large sample of adults.

May/June 2019

The cilia on ciliated epithelial cells vary in length, both within one person and from one individual to another. Samples of ciliated epithelial tissue were taken from the airways of healthy people, and the mean cilia length was found for each person. The participants in the investigation were divided into two categories: • people who had been exposed to a harmful environmental factor • people who had not been exposed to a harmful environmental factor. The findings are shown in Fig. 3.1.

May/June 2019

Yeast cells are unicellular eukaryotes. They react to changes in the concentrations of the sugars galactose and glucose by turning on genes through transcription factors. Fig. 5.1 shows the events that occur when galactose is present and glucose is absent in the external environment.

May/June 2020

Yeast cells sense changes in the glucose concentration around them by using transcription factors to turn genes off. When glucose is present: Mig1 transcription factors attach to the promoters of five genes, and once Mig1 is bound to the promoters, transcription of these genes is switched off (repressed). The genes repressed by Mig1 code for five enzymes that enable yeast cells to metabolise the sugar galactose when glucose is not present.

May/June 2020

Yeast cells are unicellular eukaryotes that alter which genes are switched on or off in response to whether different sugars are present. Fig. 5.1 summarises one example of this. When glucose is present, the following happens: yeast cells use constitutively expressed enzymes to metabolise glucose; the Mig1 transcription factor $A$ attaches to promoter $B$; transcription of gene $C$ is prevented; enzyme $D$ is no longer produced. When galactose is present and glucose is absent, a different sequence occurs: the Msn2 transcription factor $E$ attaches to promoter $B$; transcription of gene $C$ is activated; enzyme $D$ is made and assists in converting galactose to glucose. Gene $F$ encodes the Mig1 transcription factor, $A$. Gene $G$ encodes the Msn2 transcription factor, $E$.

May/June 2020

Explain how the different forms of gene mutation can influence the phenotype and outline the effects on a person’s phenotype of the mutant alleles responsible for Huntington’s disease.

May/June 2021

The bacterium $Escherichia\ coli$ can metabolise glucose or disaccharides such as lactose. Lactose must first be hydrolysed by the enzyme $\beta$-galactosidase, producing glucose and galactose, and these can then be used by $E.\ coli$. Production of $\beta$-galactosidase is regulated by a DNA sequence known as the $lac$ operon.

May/June 2022

Sweet peas are garden plants whose height differs. • Tall sweet peas reach 200 cm in height. • Dwarf sweet peas reach 30 cm in height. • Tall sweet peas have a dominant $Le$ allele. • Dwarf sweet peas are homozygous for the recessive $le$ allele.

May/June 2023

Gene expression inside a cell is regulated. If a gene is expressed (switched on), transcription of that gene occurs. If a gene is not expressed (switched off), transcription does not take place. Environmental changes can lead certain genes to be turned on or turned off.

May/June 2023

Genes play a part in controlling the overall phenotype of an organism.

May/June 2024

An operon is a DNA segment in prokaryotes.

May/June 2025

Cultivated rice, Oryza sativa, is commonly grown in waterlogged fields that are covered by floodwater.

Oct/Nov 2011

Penicillin belongs to the antibiotics known as β‑lactams, and all of them act on bacteria in the same way. Describe how penicillin kills non-resistant bacteria.

Oct/Nov 2012

Fig. 4.1 illustrates the stages of spermatogenesis in a mammal.

Oct/Nov 2012

Penicillin is part of the antibiotic group called $\beta$ lactams, and every member of this group works in the same manner against bacteria.

Oct/Nov 2012

Bread wheat, Triticum aestivum, is a hexaploid that originated from diploid wild grasses.

Oct/Nov 2015

The $\textit{lac}$ operon is a DNA segment found in the genome of $\textit{Escherichia coli}$. The structural genes in the $\textit{lac}$ operon are fully expressed only when the bacteria experience high lactose concentrations. Fig. 6.1 is a diagram of the $\textit{lac}$ operon and an adjacent section of the $E.\ coli$ genome.

Oct/Nov 2018

Gibberellin plays a part in the germination of barley seeds. In one investigation, aleurone layers were taken from barley seeds. One group was exposed to gibberellin, whereas the other group received no gibberellin treatment. The rate at which amylase enzyme was made by the aleurone layers was tracked for 15 hours. The findings are presented in Fig. 7.1.

Oct/Nov 2019

Several mutations influence the synthesis of fetal haemoglobin, HbF, and normal adult haemoglobin, HbA. • The HbA allele encodes the normal β-globin polypeptide of haemoglobin. • The HbS allele, which results from a base substitution mutation, encodes an abnormal β-globin polypeptide. • This base substitution changes the amino acid glutamine, with a polar R group, into valine, with a non-polar R group, in the polypeptide. The abnormal haemoglobin molecules (HbS) assemble into fibres at low partial pressures of oxygen ($p_{O_2}$). The fibres make red blood cells sickle shaped and the cells can obstruct blood capillaries. People whose adult haemoglobin molecules are all abnormal (HbS) have sickle cell anaemia. This is a painful chronic condition and it can be life-threatening.

Oct/Nov 2020

Section B. Answer any one question.

Oct/Nov 2020

Several mutations affect how fetal haemoglobin, HbF, and normal adult haemoglobin, HbA, are made. • The HbA allele encodes the normal $\beta$-globin polypeptide of haemoglobin. • The HbS allele, which results from a base substitution mutation, encodes an abnormal $\beta$-globin polypeptide. • Because of the base substitution, the amino acid glutamine, with a polar R group, is substituted by valine, with a non-polar R group, in the polypeptide. When the partial pressure of oxygen is low ($pO_2$), the abnormal haemoglobin molecules (HbS) become fibres. These fibres make red blood cells sickle-shaped, and the cells may obstruct blood capillaries. People whose adult haemoglobin is entirely abnormal (HbS) have sickle cell anaemia. This is a long-term painful disorder that may be fatal.

Oct/Nov 2020

As barley seeds germinate, amylase is made.

Oct/Nov 2021

In prokaryotes, the lac operon is made up of a set of structural genes that are controlled by the same regulatory system and transcribed as a single unit. A further operon present in prokaryotes is the trp operon. Fig. 6.1 gives a summary of the organisation and control of the trp operon.

Oct/Nov 2022

Plant physiology is influenced by environmental factors such as light intensity.

Oct/Nov 2023

The lac operon occurs in the genome of the bacterium Escherichia coli. If glucose is unavailable, lactose in the extracellular environment causes the genes of the lac operon to be expressed. This increases the uptake and metabolism of lactose. If glucose and lactose are both present in the extracellular medium, lactose is stopped from entering the bacterial cell. Fig. 3.1 illustrates what occurs when glucose enters a bacterial cell. • Glucose enters the bacterial cell via transport protein, A. • B is an enzyme located in the cytoplasm of the bacterial cell. • B catalyses the phosphorylation of glucose to form glucose 6-phosphate.

Oct/Nov 2025