Stem cells, gene expression and phenotype: exam questions

Answer: B

Answer: B

• stem cell: an undifferentiated cell (1)
• that can divide by mitosis / self-renew and differentiate into specialised cells (1)
• pluripotent: can differentiate into almost any / many types of cell in the body, but not extra-embryonic (placental) cells (1)
• Max 3

• scientists provide evidence about the potential benefits, for example treating diseases such as diabetes / spinal injury (1)
• and about the risks, for example tumour formation / rejection (1)
• ethical objections: embryo destroyed / embryo has the right to life (1)
• religious / moral views differ (1)
• spare IVF embryos would be destroyed anyway (1)
• laws / regulatory bodies license and control the research (1)
• decisions use the evidence together with public opinion / ethics committees (1)
• alternatives such as adult stem cells may avoid the ethical issue (1)
• Max 6

• different genes are expressed / switched on in each cell (1)
• only these genes are transcribed to produce (active) mRNA (1)
• mRNA is translated into different proteins (1)
• proteins act as enzymes that control cell processes / determine the cell's structure (1)
• for example the pancreatic cell expresses the insulin gene, the muscle cell expresses the actin and myosin genes (1)
• Max 4

• gene is transcribed to give pre-mRNA containing introns and exons (1)
• introns are removed and exons joined (splicing) (1)
• post-transcriptional change / happens before translation (1)
• exons can be joined in different combinations / some exons left out (alternative splicing) (1)
• different mature mRNAs are made and translated into different proteins (1)
• Max 4

• DNA methylation: methyl groups added to cytosine bases (1)
• this prevents transcription / switches the gene off (1)
• histone modification: acetylation loosens the DNA so the gene can be transcribed (1)
• deacetylation / (some) histone methylation makes the DNA more tightly wound so the gene is not transcribed (1)
• the DNA base sequence is not changed (1)
• methylation pattern copied on to the new DNA after replication, so passed on to daughter cells (1)
• Max 5

• phenotype is the result of the interaction of genotype and the environment (1)
• different diets / lifestyles / environments affect gene expression, for example by epigenetic changes (1)

• gametes of man: Iᴬ and Iᴼ (1)
• gametes of woman: Iᴬ and Iᴮ (1)
• offspring genotypes IᴬIᴬ, IᴬIᴮ, IᴬIᴼ, IᴮIᴼ (1)
• probability of group B (IᴮIᴼ) = ¼ / 25% (1)

• bell-shaped / normal distribution, with most students near the middle and fewer at the extremes (1)
• modal class is 165 to 169 cm (1)
• continuous variation caused by many genes (polygenic), each with a small additive effect (1)
• plus environmental effects such as diet / nutrition (1)
• a range of heights with no distinct categories / heights are measured, not counted (1)
• Max 4