Topic 3 - Voice of the Genome

Question 1

Compare and contrast prokaryotes and eukaryotes.

Answer 1

Bacteria and cyanobacteria (photosynthetic bacteria) together make up the prokaryotae kingdom. They are smaller than eukaryotes with diameters between 0.5 and 5 um whereas eukaryotes have diameters 20um or more. Prokaryotes do not contain membrane bound organelles whereas eukaryotes do. Smaller ribosomes in prokaryotes than in eukaryotes. Prokaryotes have a flagellum whereas eukaryotes do not. Prokaryotes also have circular DNA. Additionally, not all eukaryotes have a cell wall.

Question 2

Describe the functions of the structures that are not always present in prokaryotic cells.

Answer 2

a. Plasmid – small circles of DNA
b. Capsules – A slimy layer on the surface for protection and to prevent dehydration.
c. Pili – thin protein tubes which allow bacteria to adhere to surfaces.
d. Flagellum – hollow cylindrical thread-like structure which rotates to move the cell.

Question 3

What structures are present in all prokaryotic cells?

Answer 3

a. Infolding of the cell surface membrane – site of respiration.
b. Ribosomes which are smaller than in eukaryotes.
c. Circular DNA
d. Cell surface membrane
e. Cell wall (contains peptidoglycan, a type of polysaccharide and polypeptide combined.
f. Cytoplasm

Question 4

What is the calculation for magnification?

Answer 4

Magnification = image size / actual size

Question 5

How are proteins processed after being synthesised in our body?

Answer 5

a. Once transcription of DNA to mRNA, the mRNA leaves the nucleus, proteins made on ribosomes enter rough ER (endoplasmic reticulum), protein moves through the ER assuming three-dimensional shape en route.
b. Vesicles pinched off the rough ER contain the protein, vesicles from rough ER fuse to form the flattened sacs of the Golgi apparatus, proteins are modified within the Golgi apparatus, vesicles pinched off the Golgi apparatus contain the modified protein, the vesicles then fuse with the cell surface membrane releasing protein, such as extracellular enzymes.

Question 6

Describe fertilisation.

Answer 6

a. If intercourse takes place at about the time of ovulation, sperm may meet the ovum in the oviduct. The sperm which contains a protein called antifertilizin are attracted to the ovum by chemicals released from it, a protein called fertilizin. The acrosome in the head of the sperm swells, fuses with the sperm cell surface membrane and releases digestive enzymes. These enzymes break down the follicle cells and zona pellucida of the ovum. This is called the acrosome reaction.
b. The acrosome is a type of lysosome. Lysosomes are enzyme-filled sacs found in the cytoplasm of many eukaryotic cells.
c. Once a sperm fuses with and penetrates the membrane surrounding the egg, enzymes released from lysosomes in the ovum cause the zona pellucida, to thicken, preventing any further sperm entering the egg. This is called the cortical reaction. The sperm nucleus that enters the egg fuses with the egg nucleus to produce a fertilised egg (zygote).

Question 7

Where does meiosis occur and what important roles does it have?

Answer 7

a. Meiosis produces gametes with only half the number of chromosomes, called the haploid number (n) (23 in humans).
b. Meiosis occurs in the ovaries and testes of animals, and the ovaries and anthers of flowering plants.

c. Meiosis has two important roles.
i. Firstly, it results in haploid cells, which are necessary to maintain the diploid number after fertilisation.
ii. Secondly, it helps create genetic variation among offspring.

Question 8

Describe independent assortment.

Answer 8

During meiosis only one chromosome from each pair ends up in each gamete. Either chromosome from each pair could be in any gamete. For example, an organism with six chromosomes, that is three homologous pairs XX, YY and ZZ, could form eight (2^3) combinations in its gametes.

Question 9

Describe crossing over.

Answer 9

During the first meiotic division, homologous chromosomes come together as pairs and all four chromatids come into contact. At these contact points the chromatids break and rejoin, exchanging section of DNA between non-sister chromatids. The point where the chromatids break is called a chiasma and several of these often occur along the length of each pair of chromosomes, giving rise to a large amount of variation. There is no crossing over between the sex chromosomes during meiosis.

Question 10

What is Gregor Mendel’s garden peas experiment in 1850?

Answer 10

He crossed pure breeding, homozygous, tall purple-flowered plants with short white-flowered plants. All of the offspring produced were tall with purple flowers. He went on to cross two of these plants and found that plants were produced showing all combinations of the characteristics: tall and purple, tall and white, short and purple, and short and white.

Question 11

When are genes inherited independently?

Answer 11

genes are inherited independently only if they are on separate chromosomes or are far apart on the same chromosome.

Question 12

What is the linkage of genes?

Answer 12

a. Any two genes with a locus on the same chromosome are linked together and will tend to be passed as a pair to the same gamete. This is known as linkage of genes.
b. The genes will only be separated and go into different gametes if crossing over happens between the pair of genes. If the two genes are very close together on the same chromosome, crossing over is very unlikely to happen between them. The two genes are said to be strongly linked and they will be inherited as a pair.

Question 13

How many linkage groups are inside humans and how are chromosomes numbered?

Answer 13

a. Genes on a single chromosome make up a linkage group. In humans, there are 23 linkage groups because we have 23 pairs of chromosomes.
b. Chromosomes are numbered from the longest to the shortest, so we expect chromosome 1 to contain the most genes and chromosome 22 the fewest.
c. Chromosome 23 is the sex chromosome.

Question 14

What is sex linkage?

Answer 14

a. Mammals, including humans, sex is controlled by the sex chromosomes.
b. Females are XX and males XY.
c. The Y chromosome contains genes that make a person male and very few other genes.
d. All the genes on the sex chromosomes are passed on with those that determine sex, they are sex linked.

Question 15

How is red-green colour blindness sex linked and how is it inherited?

Answer 15

a. The gene loci for the three pigments found in the eye’s cone cells are on the X chromosome.
b. Cone cell pigments are needed for colour vision.
c. If one or more cone pigments is faulty or absent, the person will not be able to see colours normally: they will have some form of colour blindness.
d. Red-green colour blindness happens when there is a mutation in one of the cone pigment genes. This could happen (very rarely) when an egg or sperm is produced in meiosis.
e. The Y chromosome has no locus for the gene.

Question 16

What is one cause of male infertility?

Answer 16

For the human zygote to develop, the gamete nuclei have to fuse and a chemical from the sperm cytoplasm is required to activate the fertilised cell. The chemical is a protein called oscillin. It causes calcium ions to move in and out of stores in the cytoplasm of the ovum. These oscillations of calcium ion concentration trigger the zygote to begin developing into an embryo. Oscillin is concentrated in the first part of the sperm to attach to the ovum and enters before the male nucleus, activating the ovum. It is though that low levels of oscillin in sperm may be linked to male infertility.

Question 17

What happens in the S phase of the cell cycle and how long does it take?

Answer 17

a. In the S phase, the cell synthesises a complete copy of the DNA in its nucleus. It also duplicates a microtubule-organising structure called the centrosome. The centrosomes help separate DNA during the M phase.
b. The S and G2 phase of most cells remain relatively constant in duration.

Question 18

How is DNA organised within chromosomes?

Answer 18

a. DNA winds around histone proteins.
b. DNA and histone proteins coil to form chromatin fibre.
c. Chromatin fibre attaches to a protein scaffold forming loops.
d. Folding the protein scaffolding produces the condensed chromosome structure seen during nuclear division.

Question 19

Describe what happens in interphase.

Answer 19

During interphase, new cell organelles are synthesised, and DNA replication occurs. By the end of interphase, the cell contains enough cell contents to produce two new cells.

Question 20

What happens in prophase?

Answer 20

a. During prophase, the chromosomes condense, becoming shorter and thicker, with each chromosome visible as two strands called chromatids. Apart from the occasional mutation, the two strands are identical copies of one another, produced by replication. They are effectively two chromosomes joined at one region called the centromere.
b. During prophase, microtubules from the cytoplasm form three-dimensional structure called the spindle. The centrioles move around the nuclear envelope and position themselves at opposite sides of the cell. These form the two poles of the spindle and are involved in the organization of the spindle fibres. The spindle fibres form between the poles. The widest part of the spindle is called the equator. The breakdown of the nuclear envelope signals the end of prophase and the start of the next stage.

Question 21

What happens in metaphase?

Answer 21

During metaphase, the chromosomes’ centromeres attach to spindle fibres at the equator. When this has been completed the cell has reached the end of metaphase.

Question 22

What happens in anaphase?

Answer 22

During anaphase, the centromeres split. The spindle fibres shorten, pulling the two halves of each centromere in opposite directions. One chromatid of each chromosome is pulled to each of the poles. Anaphase ends when the separated chromatids reach the poles and the spindle breaks down.

Question 23

What happens in telophase?

Answer 23

The last stage of mitotic division is called telophase. The chromosomes unravel and the nuclear envelope reforms, so that the two sets of genetic information become enclosed in separate nuclei.

Question 24

What happens in cytoplasmic division?

Answer 24

a. After nuclear division, the final reorganisation into two new cells occurs. This is called cytoplasmic division. In animal cells, the cell surface membrane constricts around the centre of the cell. A ring of protein filaments bound to the inside surface of the cell surface membrane is thought to contract until the cell is divided into two new cells. It has been proposed that the proteins actin and myosin, responsible for muscle contraction, may also be the proteins responsible for cytoplasmic division.
b. Instead of undergoing this constriction, plant cells synthesise a new cell plate between the two new cells.

Question 25

What is mitosis important for?

Answer 25

a. Mitosis ensures genetic consistency, with daughter cells genetically identical to each other and to the parent cell. This is achieved by: DNA replication prior to nuclear division and the arrangement of the chromosomes on the spindle and the separation of chromatids to the poles.
b. Growth and repair – some organisms can regenerate lost or damaged parts of their bodies using mitosis. Starfish can even re-grow a completely new body cell from a fragment. Mitosis also allows old and damaged cells to be replaced with identical new copies.
c. Asexual reproduction – many organisms reproduce without producing gametes.

Question 26

What happens in embryonic development?

Answer 26

a. After a human zygote has undergone three complete cell cycles, it consists of eight identical cells. Each of these cells is said to be totipotent as it can develop into a complete human being. This is what happens when identical twins (or triplets or even quadruplets) form – such twinning can occur up to 14 days after conception.
b. By five days after conception, a hollow ball of cells called the blastocyst has formed. The outer blastocyst cell layer goes on to form the placenta. The inner cell mass of 50 or so cells goes on to form the tissues of the developing embryo. These 50 cells are known as pluripotent embryonic stem cells. Each of these cells can potentially give rise to most cell types, though they cannot give rise to all of the 216 different cell types that make up an adult human body.

Question 27

What is the calculation for mitotic index?

Answer 27

(prophase + metaphase + anaphase + telophase) / total number of cells

Question 28

Describe totipotency.

Answer 28

Has the potential to give rise to all cell types.

Question 29

Describe pluripotency.

Answer 29

Has the ability to give rise to most cell types, though they cannot each give rise to all of the 216 different cell types that make up an adult human body.

Question 30

Describe multipotency.

Answer 30

a. Some cells retain a certain capacity to give rise to a variety of different cell types.
b. For example, neural stem cells can develop into the various types of cells found in the nervous system.
c. While blood stem cells located in bone marrow can develop into red blood cells, platelets and the various sorts of white blood cells (macrophages, lymphocytes, etc.). These are often called adult stem cells.

Question 31

Where is cell differentiation reversible?

Answer 31

a. Cell differentiation is irreversible in animals, but many differentiated plant cells can de-differentiate and then develop into a complete new plant.
b. Totipotency of plant cells allow plants to be reproduced using plant tissue culture. Small pieces of a plant, known as explants, are surface-sterilised and then placed on a solid agar medium with nutrients and growth regulators. the cells divide to form a mass of undifferentiated cells known as a callus. by altering the growth regulators in the medium, cells of the callus can be made to differentiate to form small groups of cells that are very similar to plant embryos. These embryos develop into complete plants that are genetically identical clones.

Question 32

Describe one problem associated with stem cells in medicine?

Answer 32

a. Even if scientists manage to get the stem cells to develop into the right sort of tissue, the tissue might end up being rejected by the immune system of the person given the transplant.
b. It might be possible to get around the problem of rejection using drugs the prevent the patient from rejecting any transplanted organ or by tissue-typing.

Question 33

How does therapeutic cloning work?

Answer 33

a. One of the patient’s diploid cells is removed – this could simply be a cell from the base of a hair or any other suitable tissue.
b. This cell, or its nucleus, would then be fused with an ovum from which the haploid nucleus had been removed.
c. The result would be a diploid cell rather like a zygote. This process is known as somatic cell nuclear transfer (a somatic cell is any diploid body cell).
d. This cell could then be stimulated to divide by mitosis in the same way as the cell that gave rise to Dolly the cloned sheep.
e. After about five days a blastocyst would develop. Stem cells could then be isolated from this and encouraged to develop into tissues. This procedure results in cell lines, and perhaps eventually organs for transplantation, which are genetically identical to the patient from whom the original diploid cell was taken.

Question 34

What are some ethical concerns about the use of stem cells?

Answer 34

a. The use of any embryo for research purposes is unethical and unacceptable on the grounds that an embryo should be accorded full human status from the moment of its creation.
b. Some argue that the embryo requires and deserves no particular more attention whatsoever.
c. Others accept the special status of an embryo as a potential human being yet argue that the respect due to the embryo increases as it develops and that this respect, in the early stages in particular, may properly be weighed against the potential benefits arising from the proposed research.

Question 35

When are regulatory authorities needed in accordance to the use of human embryos?

Answer 35

Until 2001, UK law only allowed the use of human embryos where the HFEA considered their use to be necessary or desirable:
i. to promote advances in the treatment of infertility.

ii. to increase knowledge about the causes of congenital disease.
iii. to increase knowledge about the causes of miscarriage.
iv. to develop more effective methods of contraception.
v. to develop methods for detecting gene or chromosome abnormalities in embryos before implantation.

Question 36

How did the Dolly sheep experiment work and how did the creation of Dolly support the idea that all the genetic information for making a complete organism is present in every single cell, including those in an adult with specialised functions?

Answer 36

a. Firstly, there is a mammary cell donor sheep, whose mammary cells are grown in culture.
b. Next there is an egg cell donor sheep, an egg cell from the ovary has its nucleus removed.
c. The mammary cells and egg cell from ovary with removed nucleus are fused. So, nucleus from mammary cell is inside egg cell minus its nucleus. This is grown in culture.
d. An early embryo forms which is implanted in uterus of a third sheep (which is known as the surrogate mother). An embryo develops known as a lamb (dolly) which is chromosomally identical to mammary cell donor.
e. The adult cell providing the genetic information to create Dolly was a specialised mammary gland cell; the successful birth of Dolly suggests that the cell must have contained all the information for making a complete organism.

Question 37

What is meant by differentiation?

Answer 37

a. Cells become specialised for one function or a group of functions.
b. Cells become specialised because only some genes are switched on and produce active mRNA that is translated into proteins within cells.

Question 38

Describe the epigenome.

Answer 38

a. The epigenome influences which genes can be transcribed in a particular cell.
b. DNA is wrapped around histone proteins and both the DNA and histones have chemical markers attached to their surface. These chemical markers make up the genome.
c. The attachment of, for example, methyl groups to the DNA of a gene, usually to cytosine, prevents transcription to mRNA, by stopping RNA polymerase binding.
d. The modification of histones by addition of, for example, methyl or acetyl groups, affects how tightly the DNA is wrapped around the histone. When wound tightly, the genes are inactive: they cannot be transcribed to mRNA. The gene therefore cannot make protein: it is ‘switched off’.
e. During DNA replication, the epigenetic markers are copied with the DNA so that the correct set of genes remain active. Although all the active genes may not all be transcribed continuously.

Question 39

Describe the lac operon model.

Answer 39

a. French geneticists Jacob and Monod studied the control of genes in E. coli which only produce the enzyme B-galactosidase to break down the carbohydrate lactose. When lactose is present in the surrounding medium. This enzyme converts the disaccharide lactose to the monosaccharide’s glucose and galactose.
b. When lactose is not present in the environment, a lactose repressor molecule binds to the DNA and prevents transcription of the B-galactosidase gene. The lactose repressor binds to the operator gene and stops the B-galactosidase gene being expressed. The RNA polymerase cannot bind to the DNA promoter region.
c. When lactose is present in the environment it binds to the repressor, the repressor molecule is prevented from binding to the DNA and the B-galactosidase gene is expressed. mRNA coding for B-galactosidase is transcribed, and translation of this mRNA produces the enzyme.
d. The operator and genes associated with it are known as an operon, in this case the lac operon.

Question 40

What switches transcription of an individual gene on or off in eukaryotes?

Answer 40

a. Genes in uncoiled, accessible regions of the eukaryote DNA can be transcribed into mRNA. The enzyme RNA polymerase binds to a section of the DNA adjacent to the gene to be transcribed. This section is known as the promoter region. Only when the enzyme has attached to the DNA will transcription proceed.
b. The gene remains switched off until the enzyme attaches to the promoter region successfully. The attachment of a regulator protein is usually also required to start transcription.
c. Transcription of a gene can be prevented by protein repressor molecules attaching to the DNA of the promoter region, blocking the attachment site. Additionally, protein repressor molecules can attach to the regulator proteins themselves, preventing them from attaching. In either case, the gene is switched off: it is not transcribed within this cell.

Question 41

How are cells organised into tissues?

Answer 41

a. Specialised cells can group themselves into clusters, working together as a tissue.
b. Cells have specific recognition proteins known as adhesion molecules on their cell surface membranes which help cells to recognise other cells like themselves and stick to them.
c. A small part of each recognition protein is embedded in the cell surface membrane; a larger part extends from the membrane. This exposed section binds to complementary proteins on the adjacent cell. The particular recognition proteins synthesised by a cell determine which cells it can and cannot attach to. Synthesising tissues from many cells complementary binding together through adhesion molecules.
d. In tissues, cells also interact with the extracellular matrix, a network of molecules secreted by cells. In some tissues, the extracellular matrix is a major component of the tissue: for example, in cartilage.

Question 42

What is a tissue, organ and organ systems?

Answer 42

a. Tissue – a group of specialised cells working together to carry out one function. For example, muscle cells combining to form muscle tissue and epithelial cells forming epithelial tissue.
b. Organ – a group of tissues working together to carry out one function. For example, muscle, nerve and epithelium work together in the heart.
c. Organ systems – A group of organs working together to carry out a particular function. For example, the circulatory system.

Question 43

What is apoptosis (programmed cell death)?

Answer 43

a. During development, an animal must lose some cells by apoptosis. There is a small group of cell ‘suicide’ genes and when they are expressed this causes the nucleus and cytoplasm to fragment.
b. Genes that prevent death are expressed in most cells and during development, so cells have to switch on their ‘suicide’ genes in order to die.

Question 44

What is a phenotype?

Answer 44

a. The characteristics of an organism, such as its size, shape, blood group or sex, are known as its phenotype.
b. Differences in phenotype are caused by differences in: genetic make-up or genotype or the environment in which an individual develops.

Question 45

What is discontinuous variation and continuous variation?

Answer 45

a. Discontinuous variation is where characteristics fall into discrete groups with no overlap and are mainly only affected by genotype. For example, a person’s blood group (group A, B, AB or O) is only controlled by genes that code for the glycoprotein on the surface of red blood cells.
b. Characteristics that are affected by both genotype and environment often show continuous variation. Characteristics that show continuous variation are controlled by genes at many loci, known as polygenic inheritance. They are also controlled by the environment, either directly or by influencing gene expression. They have a bell-shaped graph. For example, a person’s height will be continuous variation.

Question 46

Define polygenic inheritance.

Answer 46

a. When a number of genes are involved in the inheritance of a characteristic. Such inheritance involves interaction of alleles at many loci.
b. The greater the number of loci of alleles contributing towards a gene the greater the range of the phenotype (the greater the number of classes).

Question 47

Define multifactorial.

Answer 47

Conditions where several genetic factors and one or more environmental factors are involved are said to be multifactorial.

Question 48

Describe changes around height.

Answer 48

a. Overall, there has been an increase in human height over recent generations.
b. Taller men have more children.
c. Greater movement resulting in less inbreeding.
d. Better nutrition and improved health.
e. End of child labour.
f. Better heating.
g. Better clothing.

Question 49

Describe the production of melanin.

Answer 49

a. Melanin is made in melanocytes (found in the skin and at the root of the hair in the follicle). Melanocytes are activated by melanocyte-stimulating hormone (MSH), the surface of melanocytes have receptors for MSH. Once activated melanocytes place melanin into organelles called melanosomes. The melanosomes are transferred to nearby skin and hair cells where they collect around the nucleus, protecting the DNA from harmful UV light.
b. People with more receptors have darker skin and hair; so, have more protection against sunburn.
c. UV light increases the amount of MSH and MSH receptors on melanocytes causing the skin to darken.
d. However, hair does not appear darker because although more melanin is produced, UV light causes chemical and physical changes to melanin and other proteins in hair cells. Hair lightens due to destruction of melanin by UV light.

Question 50

Describe colour in animals.

Answer 50

Tyrosinase catalyses tyrosine into melanin. Animals such as Himalayan rabbits and Siamese cats have mutated alleles for tyrosinase. The enzyme is unstable and inactivated at normal body temperature. This results in darker coloured paws, tails and ears.

Question 51

Describe nature and nurture as influences for epigenetic changes.

Answer 51

a. Nature: environment can trigger changes, such as medicines or drugs.

b. Nurture: behaviour can have an effect on whether a gene is expressed.
i. A “good” mother and a “bad” mother switch offspring (baby rabbits). The good mother is high in grooming, which leads to no methylation, which expresses a gene. The bad mother is low in grooming, which results in methylation and the gene is not expressed.

Question 52

Define cancer.

Answer 52

a. Rate of cell multiplication is faster than cell death. Leading to the growth of tumours, often in tissues with a high rate of mitosis, such as the lung, bowel, gut or bone marrow.
b. Cancers are caused by damage to DNA.

Question 53

Describe oncogenes.

Answer 53

Oncogenes code for proteins stimulating transition in the cell cycle. Mutations in oncogenes may lead to excessive or quicker cell division.

Question 54

Describe tumour suppressor genes.

Answer 54

a. Produce suppressor proteins which stop the cycle.
b. Loss of tumour suppressor proteins has been linked with different variations of cancers.
c. One example is p53 which stops the cell cycle by inhibiting the enzymes at G1/S transition, preventing the cell from copying its DNA. In cancer cells, a lack of p53 means the cell cannot stop entry into the S phase. Such cells have lost the control of the cell cycle. Loss of tumour suppressor proteins has been linked to skin, colon, bladder and breast cancers.

Question 55

Describe general risk factors for cancer.

Answer 55

Genetic predisposition.
Smoking.
Exposure to UV light.
Poor diet.
Viral infections.
Exposure to asbestos.