2.3.3 Describe the ultrastructure of an animal (eukaryotic) cell (nucleus, nucleolus, ribosomes, rough and smooth endoplasmic reticulum, mitochondria, centrioles, lysosomes, and Golgi apparatus) and recognise these organelles from EM images.

Ultrastructure is the name for the fine structure that is revealed when using a powerful microscope such as an electron microscope. 

Organelles found in eukaryotic cells:

• rER is a series of single, flattened sacs (cisternae) enclosed by a single membrane, there are ribosomes on their surface membrane, they store and transport proteins.
• sER is a series of single, tubular flattened sacs enclosed by a single membrane, involved in synthesis and transport of steroids and lipids.
• nucleus is surrounded by a double membrane (nuclear envelope), and has a dark-staining area, called the nucleolus (the only organelle without a cell membrane) within the nuclear envelope, as well as nuclear pores. It is the site of DNA and mRNA synthesis.
• centrioles are two hollow cylinders arranged at right-angles to each other, they are only found in animal cells, don't have a membrane as part of their structure, each bundle is made up of nine microtubules. The produce spindle fibres during cell division.
• lysosomes are enclosed by a single membrane, and contains digestive (hydrolytic) enzymes to digest materials within cells.
• Golgi apparatus are a series of single, curved sacs (cisternae) enclosed by a single membrane, each sac is smaller than the previous one, and many vesicles cluster around the Golgi apparatus. It modifies proteins such an attachment of carbohydrates to form glycoproteins and extracellular enzymes.  

mitochondria surrounded by a double membrane (envelope), inner membrane filed into finger-like projections called cristae (singular, crista), central area contains a jelly called the matrix and floating in the matrix are 70’s ribosomes and DNA as a loop (circular DNA). It is the site of aerobic respiration.




animal cell under electron microscope

mitochondria under electron microscope

chloroplast

2.3.2 Distinguish between eukaryotic and prokaryotic cells in terms of their structure and ultrastructure.

Key Terms 

Eukaryotic Cell: A cell with a nucleus (that contains genetic material and surrounded by a double membrane (or envelope)) and membrane-bound organelles in its cytoplasm ( organelles surrounded by one or two membranes).

Prokaryotic Cell: A cell that does not (and never did) have a nucleus or membrane bound organelles in its cytoplasm.

Cell Ultrastructure: The structure of cells seen using a microscope with high degree of resolution, usually a transmission electron microscope.

Differences between eukaryotic and prokaryotic cells

feature	prokaryotic cell	eukaryotic cell  animal cell                              plant cell
nucleus w envelope	absent	present	present
membrane-bound organelles	absent	present (e.g. nucleus, mitochondria, Golgi apparatus, rER)	present
dna found as	a loop (a single chromosome/double strand loose in the cytoplasm forming a circular loop/ nucleoid) no histones	separate strand (double strand linear DNA incorporated with histone proteins in the chromosomes)	separate strand
slime capsule	sometimes present	never present	never present
flagella	simple (no micro tubes) (if present)	complex ( has micro tubes)(if present)	absent
cell wall	present (bacterial) made of murein/ peptidoglycan	absent	preset (plants and algae) (cellulose)
relative size of cell	small	medium	large
ribosomes	70’s/ smaller ribosomes occur free in the cytoplasm.	80’s/ larger ribosomes occur in the cytoplasm or bound to the endoplasmic reticulum to form rER. also present in nuclear envelope
site of aerobic respiration	mesosome (if present)	mitochondria

1.4.20 Identify and discuss the social and ethical issues related to genetic screening from a range of ethical viewpoints.

Ethical Issues

Right to life of the fetus, abortion is murder (in the event of a positive diagnosis), who has the right to decide if the test should be done, and who has the right to make decisions for the foetus if the test is positive.

Social Issues

Cost of brining a disabled baby/cost of procedure so parents may disagree, some other genetic abnormalities may be found which may lead to discrimination in insurance or employment, issues relating to confidentiality of parents (e.g. parental DNA does not match)

1.4.19 Explain the uses of genetic screening: identification of carriers, preimplantation genetic diagnosis and prenatal testing (amniocentesis and chorionic villus sampling) and discuss the implications of prenatal genetic screening.

Genetic Screening

- Newborns could be tested for a faulty allele of the CF gene, but the CF gene has many mutations and no test can cover all of them, so a negative result could be false. 

- DNA testing of adults to identify carries: a couple were both are carriers have a 1 in 4 chance of having a baby with the genetic disorder 

- If a couple have been identified as carriers yet they wish to have a baby, they might opt for prenatal screening, screening the fetus in the uterus to detect genetic defects:

1. Amniocentesis: some of the amniotic fluid is removed from the amniotic sac of the mother using a fine needle inserted in the abdomen, the fetal cells present in the amniotic fluid are obtained and their DNA is analysed and used to detect defective genes/alleles.
2. Chorionic villus sampling: cell sample from embryonic tissue is taken from the developing placenta either using a needle in the abdomen or a catheter in the vagina, the DNA from the cell is isolated and used to detect defective genes.

- Implications: risk of abortion or harm to fetus, mental and emotional issues surrounding the birth of a disabled baby, being prepared for a baby born with CF or other genetic disorders.

- Preimplantation Genetic Diagnosis (PIDG): embryos created through in vitro fertilisation (outside the body) are tested to see of they carry they faulty allele, only those which do not are implanted into the mother’s uterus. 

1.4.18 Describe the principles of gene therapy and distinguish between somatic and germ line therapy.

- Gene therapy is the insertion of a normal allele of a gene into cells to replace a faulty allele that causes an inherited disorder. 

- This could be done in the very early embryo (germ line therapy) or in the affected body part or parts (somatic therapy).

Somatic Therapy

Identify the gene involved, make copies of the normal allele and insert into a vector ( e.g. viruses/ liposomes), use the vector to insert the allele into the target cells. 

Germ Line Therapy

- Involves altering the germ cells - the reproductive cells - of the body of very early embryos immediately after in vitro fertilisation, so that the faulty genes are no longer passed on.

Differences

Germ line affects gametes; somatic affects body cells; germ line can be passed onto the next generation; somatic gene therapy cannot be passed onto the next generation.

1.4.17 Explain how the expression of a gene mutation in people with cystic fibrosis impairs the functioning of the gaseous exchange, digestive and reproductive systems.

- The genetic disorder cystic fibrosis is caused by mutation of the gene that codes for the CFTR protein that allows chloride ions to pass through cell membranes,: water enters the cell because chloride ions cannot leave to create the correct concentration gradient for water to move out by osmosis and dilute mucus, so mucus becomes sticky and viscous. 

- In the gas exchange system (breathing problems & lung infections), mucus accumulates in the lungs, bacteria get trapped in mucus, increasing the possibility of infection. Mucus can also block bronchioles, which reduces the number of alveoli in contact w fresh air so reduced surface area for gas exchange.

- In the digestive system mucus blocks the pancreatic duct so digestive enzymes can’t reach the duodenum (small intestine) so food is not properly digested, leading to tiredness and difficulty gaining weight. Enzymes trapped within the pancreas causes fibrosed cysts and damage to insulin-producing cells, leading to diabetes.

- In the reproductive system ( ) in women the mucus can block the cervix preventing the entry of sperm, leading to a reduction in likelihood of pregnancy. In men the sperm duct is either missing or blocked with mucus, so sperm cannot leave the testes, leading to less sperm ejaculation than normal.

1.4.16 Explain the terms gene, allele, genotype, phenotype, recessive, dominant, homozygote and heterozygote, and explain monohybrid inheritance, including the interpretation of genetic pedigree diagrams, in the context of traits such as cystic fibrosis, albinism, thalassaemia, garden pea height and seed morphology.

- Gene: the portion of the genome that carries the information for a single protein. (in cases of protein with multiple subunits, there maybe a gene for each).

- Allele: different versions of the same gene found at a specific locus and has a different base sequence.

- Genotype: the genetic makeup of an individual. genotypes can refer to an organism’s entire genetic makeup or the allele at a particular locus. 

- Recessive: an allele that is masked in the phenotype by the presence of a dominant allele, and are expressed in the phenotype when the genotype is homozygous recessive.

- Dominant: an allele that masks the presence of a recessive allele in the phenotype, express if an individual is homozygous dominant or heterozygous.

- Homozygous: having the same allele at the same locus on both members of a pair of homologous chromosomes, also refers to a genotype consisting of two identical alleles of a gene for a particular trait. 

- Heterozygous: carrying two different alleles of a gene. 

- Monohybrid inheritance: the inheritance of just one characteristic.