SNAB Topic 3: Voice of the Genome

• What are the adaptations of an egg cell?

  • Zona pellucida = jelly-like coating that acts as a protective layer that the sperm has to penetrate.
  • Follicle cells provide protective coating.
  • Lipid droplets are a food store for a developing embryo.
• Describe the role of the Golgi Apparatus and Golgi vesicles in formation and transport of extracellular enzymes

  • Protein enters Golgi apparatus
  • Modification of protein inside Golgi apparatus
  • Protein packaged into secretory vesicles
  • Vesicles fuse with cell surface membrane: exocytosis
• Describe what happens to lysosomes after their contents have been digested.

  • Lysosome fuses with cell surface membrane
  • Contents released from the cell via exocytosis
• Describe how scientists could have determined that P. falciparum is a eukaryotic organism and not a prokaryotic organism.

  • Eukaryotic organisms have membrane-bound organelles.
  • Eukaryotic organisms contain mitochondria.
  • Ribosomes larger in eukaryotes
  • Eukaryotic DNA is linear and associated with histone proteins
  • Eukaryotic organisms do not contain plasmids
• In a human liver cell, RNA is located in the nucleus, mitochondria, and cytoplasm.
• What happens to lactate in liver cells?

  • Lactate converted to pyruvate
  • Pyruvate converted to glucose for use in respiration
• Amyloplasts are only found in plant cells
• Describe the features that could be used to identify the prokaryotic cells in the blood sample.

  • Presence of a cell wall
  • circular DNA/plasmids
  • Small 70s ribosomes
  • pili/flagellum
  • capsule/mesosome
• What are Golgi vesicles also known as?

  Secretory vesicles / lysosome
• Centrioles

  • Organelle made of microtubules
  • Two perpendicular to each other/meeting in middle of one.
  • T-shaped
  • Involved in spindle organisation
• Explain why the nucleus cannot be observed at the end of prophase in a eukaryotic cell.

  • Nuclear membrane broken down
  • DNA coiled into individual chromosomes
• A three-phase protocol will be used when developing the phospholipase inhibitor as a new drug. Explain the purpose of each phase of this protocol for a phospholipase inhibitor.

  • 1 - Finding safe dosage
  • 2 - To see if it is effective in treating the condition (link to question context)
  • 3 -Gather data for tests + see side effects
• Messenger RNA coding for AFGP is translated at a ribosome to produce a polypeptide. Describe how this polypeptide is then processed to make AFGP.

  • Polypeptide chain moves through ER then Golgi
  • folded in rER
  • Carbohydrate added in Golgi
  • Transported around cell in vesicle
• Cells in people with these diseases produce incorrectly folded enzyme molecules. Explain why enzymes that are incorrectly folded cannot carry out their function.

  • Incorrect folding = wrong tertiary structure
  • Active site cant form enzyme substrate complex
  • So cant catalyse reaction
• A newborn baby can respond to infections. Interferon is involved in the response to viral infections. (i) The influenza virus can be lethal to mice. The effects of interferon on influenza infection in mice was investigated. Mice were infected with influenza virus and then given interferon. The results of the investigation are shown in the table.

  Increasing interferon dose increases survival time.

  because interferon inhibits viral replication inside cells.

  greater dose of interferon = fewer virus particles produced to infect other cells
• If asked to describe effects of one thing on another based on a table, mention correlation, look for significance or lack thereof between consecutive values and make comments. DO NOT EXPLAIN.
• ii) Devise an investigation to determine the effect of exposure time to Agil on the rate of mitosis in onion root tips.

  • Controlled conc of AGIL
  • Other control variable (temp)
  • Roots exposed for range of time intervals
  • ROOT TIP SQUASH PROCEDURE
  • NAME STAIN
  • Count number of cells undergoing mitosis to calculate mitotic index
• The eel egg cell is larger than the human egg cell, yet an adult eel is smaller than an adult human. Analyse the data in the table to explain why it is advantageous for the eel to have a larger egg cell.

  • Larger egg cell = more lipid droplets
  • = more energy for embryonic development
  • bigger sperm target so increased chances of fertilisation in open water
• Criticise this data set as evidence for a relationship between egg cell diameter and the mass of the adult.

  • Data set limited (few species)
  • Species from different taxonomic groups
  • No evidence that individuals used are representative
  • Reproductive strategy should be controlled
  • Other factors affecting adult mass
• Why is HCl added the root tip in the mitosis practical?

  To break down the middle lamella so cells can be separated / to allow light to pass through
• Why is a stain added to the root tip in the mitosis practical?

  To make the chromosomes visible so that different stages of mitosis can be identified.
• Explain how preventing the shortening of spindle fibres affects mitosis.

  • Sister chromatids cannot be separated
  • Anaphase cannot occur so mitosis stops at metaphase
  • Daughter cells are produced with the incorrect number of chromosomes
• It is claimed that the percentage of cells in a root tip undergoing mitosis during the day is different from the percentage during the night. Devise an investigation to test this hypothesis.

  • Collect root tips during day/night from plants grown in light or dark conditions
  • Use same source for root tips
  • Control temp
  • Toluidine blue
  • Squash under coverslip
  • Compare calculated mitotic index's
• At the start of fertilisation, many sperm cells will surround the ovum. Describe the events of fertilisation that occur after the acrosome reaction.

  • Fusion of sperm/egg cell membranes
  • Cortical granules release contents into zona pellucida
  • Zona pellucida hardens
  • fusion of haploid nuclei
• Even though both offspring are from the same father and the same mother, they may be genetically different. Explain why the offspring may be genetically different.

  • Random fertilisation (different gametes)
  • Different combination of alleles due to meiosis (crossing over / independent assortment)
• Duchenne muscular dystrophy (DMD) is a sex-linked disorderExplain why the genotype frequency for males with DMD cannot be calculated using this Hardy-Weinberg equation.

  • Males only have one allele so cannot be heterozygous.
  • HW assumes all individuals have two alleles for the gene
• If asked to test two hypotheses (H0 and H1), end two marks will be mentioning a statistical test (e.g. spearman's rank correlation) that can be used followed by mentioning comparison to the critical value.
• Root tip squash procedure:

  1. 2cm^3 sample into acetic alcohol for 10 minutes
  2. Cold distilled water
  3. Heated HCl
  4. Acetic alcohol
  5. Macerate with mounted needle
  6. Orcein ethanoic stain
  7. Squash under coverslip
• Explain why some genes show linkage and others show sex-linkage.

  • There are more genes than chromosomes
  • Linkage related to genes located on the same non-sex chromosome
  • Sex-linkage related to genes on sex chromosomes
• Explain how crossing over and independent assortment can produce gametes with new combinations of the alleles shown on the diagram of the two pairs of chromosomes.


  Crossing over: chromatids produced with different combinations of alleles (aB/Ab)

  Independent assortment: different combinations of chromosomes 1 and 2 are produced so A and B could be in same gamete as C or c.
• Explain how crossing over may differ in sex chromosomes.

  • Crossover unlikely to form between X and Y chromosome
  • as they are not homologous / have different lengths
• Describe how the acrosome is involved in the digestion of the zona pellucida.

  • Membrane of acrosome fuses with plasma membrane of sperm
  • Releases enzymes from acrosome
  • by exocytosis
• (ii) Explain why genes found on the sex chromosome pair have a pattern of inheritance that is different from genes found on other chromosome pairs.

  • X chromosome carries genes not present on Y chromosome
  • Males only have one copy of some genes
  • If only one allele is inherited, it will be expressed
• Deduce the effect of the P21 gene on the cell cycle when cells are treated with gamma radiation.

  • Radiation can cause damage to chromosomes
  • Cells with a functioning P21 gene shows reduced cell divison
  • with faulty P21 gene cell division remains high
  • Suggests P21 prevents cell progressing through mitosis if DNA is damaged
• Analyse the data to explain how storage time affects the growth of the seedlings of variety Q.

  • Longer storage time = slower growth of seedling
  • As less cell division after increased storage time
  • because storage conditions lead to enzymes being denatured
• (i) Give the meaning of the term polygenic.

  A characteristic showing continuous variation caused by multiple genes at different loci.
• Explain how epigenetic changes affect the development of tissues in the embryo.

  • DNA is wrapped around histones.
  • Acetylation of histone unwinds chromosome
  • Methylation of DNA tightens chromosome, inhibiting transcription of DNA.
  • Therefore gene expression is altered.
• A small number of people have been identified who are resistant to HIV. They have a mutation in a gene coding for a protein in the cell membrane. (i) Deduce why this mutation makes these people resistant to HIV infection.

  The protein is a receptor in the csm of T-helper cells.

  Glycoprotein unable to bind with CD4 receptor on the host cell.

  Viral RNA cannot enter the cell.
• Stem cells can be used to repair damaged organs. Human embryos are another source of stem cells used in medical therapies. Describe the decisions that society has to make about the use of these embryonic stem cells.

  Embryonic stem cells are totipotent and can be used in a wider range of therapies.

  Source of embryonic stem cells has to be considered.

  Ethical issues as embryos have to be destroyed.

  Licenses needed.
• Describe how cells become specialised.

  • Chemical signals cause some genes to be switched on and others to be switched off.
  • Only activated genes are transcribed.
  • Synthesis of specific proteins from these genes that cause cell modification.