Adult stem cells are undifferentiated cells that are found in most animal tissues. Adult stem cells can divide by mitosis throughout their lifespan to form identical stem cells (self-renewal) or to form cells that can differentiate into the functioning cells of that tissue. Haematopoietic stem cells (HSCs) are adult stem cells that are located in the bone marrow of bones. HSCs have a role in the formation of blood cells. Fig. 4.1 is an outline summary showing the formation of some of the different types of blood cell that can be formed from HSCs. The first stage is the division of HSCs to produce progenitor cells. These cells are also able to divide by mitosis, but are not stem cells. Key = progenitor cells [Figure 4.1: Flowchart illustrating the differentiation of Haematopoietic Stem Cells (HSCs) into various blood cell types, including progenitor cells (CMP cells, GMP cells, CLP cells, MEP cells), immature red blood cells, megakaryocytes, immature neutrophils, immature monocytes, immature B-lymphocytes, immature T-lymphocytes, mature monocytes, mature B-lymphocytes, mature red blood cells, platelets, mature neutrophils, and mature T-lymphocytes (T-helper and T-killer cells). Cell types X and Y are also indicated.]

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2024). The mark or mark type is shown in brackets.

1. [1] replacing cells that are, damaged / destroyed / worn out / old;
2. [2] any two from: result of mutation; A example proto-oncogene to oncogene tumour suppressor gene inactive I mutation / mutated, as a single term short(er) / fast(er) / many / continuous, cell cycle(s) / cell divisions ; A shorter interphase A cells do not stop dividing I ref. to mitosis no contact inhibition / mitosis continues beyond space available / cell formation may spread to other nearby areas / AW ; ref. to normal (cell cycle) checkpoints not occurring ; A idea that substances needed for error checking not, available / functioning A errors not checked loss of (original / normal) function; A example A non-functional (cell cycle continues because) fault in / errors in / no response to, cell signalling; cells do not carry out apoptosis / no programmed cell death; AVP; e.g. different cell metabolism increase in telomerase (activity) telomeres do not shorten as quickly formation / presence, of blood vessels (feature of tumour) A angiogenesis lack of adhesion between cells ref. metastasis / described e.g. cells travel in, blood / lymph, to form tumours elsewhere in body I ref. to tumour spread in blood
3. [2] any two from: idea that after each, cell cycle / cell division / DNA replication, telomeres shorten ; (long telomeres means that DNA) replication can take place more times ; A allows continuous (DNA) replication context is DNA and not cell replication idea that ends of chromosomes are protected; telomeres do not contain, genes / genetic information ; A telomeres are non-coding detail; e.g. idea that after replication nucleotides at ends of DNA are lost lagging strand replication leads to unpaired nucleotides DNA polymerase cannot continue to end on lagging strand (replication) (long telomeres) allow, long life span / many divisions to occur / many mitoses ; AVP; e.g. prevents chromosome ends from joining otherwise, mitosis could not occur / may lead to cell death
4. [3] any three from: differentiation has already started / AW ; A they are not undifferentiated A GMP cells are, differentiated / specialised (compared to stem cells) no self-renewal/AW; A cannot produce a stem cell when they divide cannot form, all blood cell types / correct named ; (only) forms (immature) neutrophils and monocytes ;
5. [1] macrophage/s; I mature monocytes / granulocyte / phagocyte
6. [1] antibody/immunoglobulin ;
7. [3] mp2 and 3 need to see knowledge that self-antigens are on, self-/body/own, cells any three from: 1 idea that, next stage / after thymus, is (T-lymphocyte) release into general circulation / blood; 2 (if released) exposure to / activation by, (self-antigens on body) cells or (on exposure to self-antigens), immune response occurs against (body) cells ; ora only (T)lymphocytes that will respond only to, pathogens / foreign substances, are, left / released 3 (if released) harm to / destruction of /kills, (body) cells ; 4 need to prevent / otherwise, formation of memory T-cells; 5 AVP; e.g. need to prevent / (if released) may cause, autoimmune, disease / response (otherwise) T-helper will release cytokines to, enhance / AW, immune response against (body) cells (if cytokine released) may lead to, phagocytosis of / antibodies produced against, (body) cells (otherwise) T-killer cell will release substance to kill cells also mp3 (faulty T-lymphocytes) cannot bind to, foreign / non-self, antigens