Topic 3

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

Eukaryotic cells have mitochondria, prokaryotes do not Eukaryotic cells have a true nucleus, prokaryotes do not have a nucleus Eukaryotic cells are larger than prokaryotes DNA is linear in eukaryotes, DNA is circular in prokaryotes

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. Nucleus - Controls what enters and leaves the cell. Has a porous nuclear envelope . Site of DNA transcription. Nucleolus - Inner region of the nucleus which contains Chromatin, the genetic material of proteins and chromosomes. Ribosome - Site of mRNA translation in protein synthesis. Floats free or is attached to rER. Has an mRNA binding site. rER - System of flattened membranes in fluid space which are studded with ribosomes which modify proteins sER - System of flattened membranes in fluid space that do not have ribosomes on its surface. Synthesises lipids Mitochondria - Double membrane bound organelle which contain inner folds called cristae, cytoplasmic matrix. Site of aerobic respiration where ATP is produced. Centrioles - Hollow cylinders made from rings of microtubules. Form the mitotic spindle fibres used in nuclear division Lysosome - Single membrane bound structure which contains digestive enzymes. Used to break down unwanted structures in the cell. Golgi Apparatus - A group of fluid filled membrane bound flattened sacs which packages proteins. The proteins enter via vesicles which fuse and pinch off the membrane (exocytosis/endocytosis) 4 Explain the role of the rough endoplasmic reticulum (rER) and the Golgi apparatus in protein transport within cells and including its role in formation of extracellular enzymes. Ribosomes produce the sequence of amino acids (polypeptide chain) which is the primary structure of a protein The ribosomes are on the surface of the rER membrane The polypeptide chain enters the rER As it travels through the rER it modifies and folds itself to a 3D protein structure. The protein is then placed within a vesicle which travels to the Golgi apparatus and fuses with the membrane Whilst travelling through the Golgi, the protein is modified again (eg. more sugar chains are added or trimmed) This protein (extracellular enzyme) then is packaged into a vesicle and transported to the cell surface membrane. The vesicle fuses with the cell surface membrane releasing the protein by exocytosis. Intracellular enzymes will be synthesised at ribosomes floating freely in the cytoplasm. Not all vesicles that are pinched off the Golgi contain proteins that are extracellular, they are simply transported around to different regions of the cell. 5 Describe how the cells of multicellular organisms can be organised into tissues, tissues into organs and organs into systems.

Cells are the smallest unit of life in organisms that can replicate independently, that perform together to carry out the same function. Tissues are groups of cells that perform together to carry out the same function. Organs are a group of tissues that perform together to carry out the same function. (Organ) systems are a group of organs that perform together to carry out the same function. 6 Explain the role of mitosis and the cell cycle for growth and asexual reproduction. Human somatic cells have a chromosome number of 46 Mitosis only occurs in somatic cells Daughter cells are identical to the parent cell During mitosis, the chromosome number is kept constant division after division Therefore a diploid parent cell will have diploid daughter cells Mitosis is only ONE stage of the cell cycle

Cell Cycle Interphase G1 - Gap intervals where cell grows before DNA replication S - DNA is duplicated (chromosomes are duplicated) G2 - Cell prepares itself for division Mitosis Prophase - Centrioles move to the opposite poles of the cell. Nuclear envelope breaks down. Chromosomes condense. Metaphase - Chromosomes become lined up at the equator. Spindle fibres from the centriole join to the centromeres of the chromosomes. Anaphase - Spindle fibres pull apart two sister chromatids of each chromosome to opposite poles of the cell Telophase - The chromatids at each pole of the cell decondense and uncoil and become long and thin. Nuclear envelope reforms around each. Cytokinesis - Cytoplasm divides and two genetically identical daughter cells are formed

7 Describe the stages of mitosis and how to prepare and stain a root tip squash in order to observe them practically.1. Cut tip from growing root (e.g garlic root squash)2. Place tip on watch glass and add a few drops of HCl3. Add a few drops of Toluidine blue so chromosomes are darker and easier to see under a microscope 4. Warm the watch glass slowly through a bunsen flame 5. Place root tip on a microscopic slide and use a mounted needle to break it open and spread the cells out thinly 6. Add a few more drops of stain and place a coverslip over it 7. Squash the cover slip down gently8. Warm the slide again for a few more seconds. This intensifies the stain9. Stages of mitosis are visible under the microscope

8 Explain the role of meiosis in the production of gametes and genetic variation through recombination of alleles and genes including independent assortment and crossing over (details of the stages of meiosis are not required). Meiosis produces a diploid daughter cell (zygote) from two haploid sex cells (sexual reproduction) Production of gametes stem from cell division by Mitosis a. DNA replicates so there are two identical copies of each chromosome (chromatids)b. DNA condenses to form 2 chromosomes made of two sister chromatidsc. Chromosomes arrange themselves into homologous pairs d. First division - homologous pairs are separated, halving the chromosome numbere. Second division - The pairs of sister chromatids are separated f.

Increases genetic variationa. Crossing over - Homologous pairs of chromosomes pair up and twist around each other and swap chromatids, exchanging genetic material. This gives a different combination of alleles for the same genes. Therefore 4 new cells with different alleles are formedb. Independent Assortment - All four daughter cells from meiosis have different combinations of chromosomes. Half is maternal and the other is paternal, which are in different combinations when gametes are produced.

9 Explain how mammalian gametes are specialised for their functions. Female gamete in mammals is the ovum and the male gamete is the sperm Sperm Lots of mitochondria to provide energy for movement Flagellum to allow sperm to move towards the ovum Acrosome contains digestive enzymes to break down the zona pellucida and penetrate the egg Haploid nucleus Arrow head (I think thats what its called) - reduced friction

Ovum (egg) Cell membrane to control what enters and leaves the cell Follicle cells to form a protective coating Zona pellucida - protective layer for sperm to penetrate Cortical granules - vesicles that fuse with the plasma membrane releasing enzymes to thicken the zona pellucida Haploid nucleus

10 Describe the process of fertilisation in mammals and flowering plants (starting with the acrosome reaction in mammals and pollen tube growth in plants and ending with the fusion of the nuclei) and explain the importance of fertilisation in sexual reproduction.

Fertilisation in mammals Acrosome reaction Sperm swims towards the egg cell (in the oviduct) Sperm makes contact with the zona pellucida and releases digestive enzymes from the acrosome Enzymes digest the zona pellucida allowing sperm to move towards the cell membrane of the ovum Sperm head fuses with the cell membrane of the ovum

Cortical reaction Triggered by the sperm head fusing with the egg cell membrane Cortical granules fuse with the plasma membrane They release enzymes This causes the zona pellucida to thicken, therefore making it impenetrable to sperm Sperm enters the egg cell and tail is discarded Sperm nucleus fuses with the egg cell to form a diploid zygote

Fertilisation in plants Occurs in the embryo sac of flowering plants This is double fertilisation1) Pollen grain lands on the stigma of the flower2) Pollen grain (absorbs water and splits open) and pollen tube grows down the style 3) This style contains two male (gamete) nuclei, one tube nuclei at the tip which makes enzymes for the pollen tube to digest down the style 4) When the tube reaches the ovary it grows through the micropyle into the embryo sac within the ovule5) In the embryo sac, the tube nucleus disintegrates and tip of the pollen tube bursts releasing the two male nuclei6) Embryo sac contains an egg nucleus, and two polar nuclei 7) One male nucleus fuses with the egg nucleus to form a zygote8) The other male nucleus fuses with the two polar nuclei to make a triploid endosperm, which acts as a food store for the mature seed

Fertilisation in sexual reproduction The moment when male and female gametes fuse Each gamete contains a haploid number of chromosomes and join to create a diploid zygote This restores the chromosome number The zygote has two sets of chromosomes, one from each parent This combines genetic material from each parent, making the offspring more genetically unique

11 Explain what is meant by the terms stem cell, pluripotency and totipotency and discuss the way society uses scientific knowledge to make decisions about the use of stem cells in medical therapies (eg regulatory authorities relating to human embryo research, ability of stem cells to develop into specialised tissues, potential sources of stem cells, who could benefit from the therapies, procedures to obtain stem cells and their risks). Stem Cell An undifferentiated cell which has the potential to differentiate into a specialised cell. Pluripotency Cells with the ability to differentiate into many but not all cell types. (except extraembryonic cells) Multipotent Cells with the ability to differentiate into some but not all cell types. Totipotency - The ability for a cell to differentiate into all cell types and potentially form a complete human being.

Use of stem cells in medical therapies. Ability of stem cells to develop into specialised tissues As embryo develops, cell becomes more differentiated. Most lose their capacity to develop into a wide range of cells. Totipotent stem cells can be used to replace damaged tissues Potential sources of stem cells Embryonic stem cells -> after 3 cell cycles, there are 8 totipotent cells. 5 days after conception -> Blastocyst forms. Within the blastocyst, approx. 50 cells are pluripotent embryonic cells. As embryo develops, cell becomes more differentiated. Most lose their capacity to develop into a wide range of cells. Adult stem cells in Bone marrow

Regulatory authorities relating to human embryo research. UK law allows embryo stem cell research to be used Treatment of infertility Causes of congenital disease Miscarriage Contraception Detecting gene or chromosome abnormalities Look at proposal of research to see if it is carried out for a good reason Make sure fully trained and licensed staff carry out the research Produce guidelines and codes of practice Provide advice and information to governments and professionals

Those who benefit from therapies involving stem cells Transplant patients pluripotent stem cells can be used to grow organs (e.g. Heart) helps reduce the waiting time for a donor transplant Therapeutic cloning Diploid cells taken out and fused with an ovum to produce a diploid cell like a zygote. This would be stimulated for nuclear division. Patients who are blind or suffering spinal cord injuries - can replace damage eye tissue and nervous tissue and improve their quality of life

Procedures and risks to obtain stem cells Stem Cell transplant injection or transfusion of stem cells into the body to repair damaged or diseased stem cells. RISKS: The rise of new cancers, death, infections, stem cell failure

Ethics from use of embryonic stem cells Destruction of embryos that are viable are unethical The moment of fertilisation is when an individual is formed and they have a right to life Unfertilised embryos are more ethical to use as they are made from egg cells that havent been fertilised by sperm Only use adult stem cells as they do not involve destroying embryos Have to take into account everyone vies when making decisions about scientific work

12 Describe how totipotency can be demonstrated practically using plant tissue culture techniques. 1. Single cell is taken from growing area of a plant (e.g. root or shoot)2. Cell is placed in growth medium (e.g agar) with growth hormones 3. Growth medium is sterile 4. Plant cell grows and divides into mass of unspecialised cells. 5. If under the right conditions, it will divide into specialised cells and differentiate into an entire plant6. This shows totipotency 13 Explain how cells become specialised through differential gene expression, producing active mRNA leading to synthesis of proteins, which in turn control cell processes or determine cell structure in animals and plants (details of transcription factors are not required at AS). The template strand of DNA used in transcription to form mRNA contains genes from the original DNA Some of the genes are active and some of the genes are inactive Only the active genes are transcribed The active mRNA strand is translated This forms a polypeptide Forms proteins. These proteins modify the cell (structure and control cell processes) Changes cause cell produced to differentiate 14 Explain how a phenotype is the result of an interaction between genotype and the environment (eg animal hair colour, human height, monoamine oxidase A (MAOA) and cancers), but the data on the relative contributions of genes and environment is often difficult to interpret.

Phenotype The physical characteristics expressed by an organism, due to the interaction between its genotype and environment.Animal Hair (skin) colour Dark pigment in skin is called Melanin. To synthesise melanin, animals use the enzyme tyrosinase. This catalyses the conversion of tyrosine (amino acid) into melanin. Melanin is produced in melanocytes. Melanocytes are activated by MSH (melanocyte- stimulating hormone) MSH binds to receptors on the melanocyte membrane. Melanocytes are activated and place melanin in melanosomes. Transferred to nearby skin cells and accumulate around the nucleus to protect the DNA from UV light. Ultraviolet (UV) light increases the amount of MSH produced and the number of receptors on the melanocyte surface. People with darker skins have more MSH receptors. Sometimes the tyrosinase enzyme is unstable (mutant alleles), and is inactivated at body temperature (produces white hair) and at cooler temperatures.Human height Average height has increased over recent years. Genes for height are inherited, however they may be affected by diet (e.g. malnutrition)MAOA (Monoamine Oxidase A) MAOA is an enzyme that breaks down neurotransmitters in the brain High levels of MAOA decrease the build-up of neurotransmitters (associated with mood) This reduces aggression.Cancer Occurs when the rate of cell multiplication is faster than the rate of cell death. Cell cycle goes out of control -> leads to the formation of a tumour (abnormal mass of cell) Common places of tumour growth (places with a high rate of mitosis) Bone marrow Lung Gut Bowel Cervix Caused by damage to DNA through Asbestos, UV light, Carcinogens (chemicals) If a tumour is not removed, cancer cells can spread to other parts of the body, through the bloodstream and lymphatic systems, causing new cancers in organs (metastasis) DNA is copied incorrectly in gamete formation leads to an inherited cancer. Genes that trigger cancer: Oncogenes Code for proteins that stimulate the cell cycle. Mutations in these genes can lead to the cell cycle being continually active and causes excessive cell division. Tumour suppressor genes (p53) Produces suppressor proteins which stop the cell cycle. Mutations that inactivate these genes causes the cell cycle to continue cell division and not stop. Cancer cells have a lack of p53 -> cell cycle cannot stop entry into the S phase, loss of control of the cell cycle. Environmental causes Ultraviolet light Exposure to asbestos Virus Viral DNA contains oncogenes from host which is transferred to potential cells. Prevention: No smoking, eat fresh fruit and veg antioxidants (free radicals)15 Explain how some phenotypes are affected by alleles at many loci (polygenic inheritance) as well as the environment (eg height) and how this can give rise to phenotypes that show continuous variation Polygenic inheritance (loci) A single characteristic that is controlled by more than 2 genes. Increasing the number of genes controlling a trait, increases the number of phenotype combinations Until the number of phenotypes to which an individual can be assigned are no longer discrete, but continuous. Continuous variation - There is an extended range of data, and there are no distinct categories (e.g. human height). Has a normal distribution (many cluster around the mean with a small number of outliers) Affected by the environment. Continuous variable Variables that are controlled by many genes (polygenic) Discontinuous variation Discrete or distinctive categories. (polygenic/monogenic) Unaffected. (e.g Blood group) Gene locus The location of a gene on a chromosome.