2.2 - All cells arise from other cells Flashcards
Describe binary fission in bacteria. (3)
- Replication of (circular) DNA
- Replication of plasmids
- Division of cytoplasm (to produce daughter cells)
Suggest and explain how two environmental variables could be changed to increase the growth rate of these cells (bacteria cells). (4)
- Increased (concentration of) glucose
- Increased respiration
or - Increased (concentration of) oxygen
- Increased respiration
or - Increased temperature
- Increased enzyme activity
or - Increased (concentration of) phosphate
- Increased ATP/DNA/RNA
or - Increased (concentration of) nucleotides
- Increased DNA synthesis
Name the fixed position occupied by a gene on a DNA molecule. (1)
- Locus/loci
Name the first three phases of mitosis. Describe the role of the spindle fibres and the behaviour of the chromosomes during each of these phases. (5)
- (In) prophase, chromosomes condense
‘accept chromatin for chromosomes and for condense shorten and thicken’ - (In) prophase OR metaphase, centromeres attach to spindle fibres
- (In) metaphase, chromosomes/pairs of chromatids at
equator/centre of spindle/cell - (In) anaphase, centromeres divide
- (In) anaphase, chromatids (from each pair) pulled to (opposite) poles/ends (of cell)
‘accept for ‘chromatids’, chromosomes but reject
homologous chromosomes’ - (In) prophase/metaphase/anaphase, spindle fibres shorten
Give one reason why trout eggs produced by meiosis are genetically different. (1)
- Independent segregation
- Crossing over
- The offspring produced from farmed trout are sterile. Suggest and explain why. (2)
- Too many/extra set/three copies of chromosomes
‘accept 1 paternal, 2 maternal’ - (Homologous) chromosomes do not pair
/(Homologous) chromosomes do not separate (evenly)
‘accept divide for separate’ - (So) no meiosis
Suggest why preventing the formation of spindle fibres stopped the cell cycle. (2)
- Chromosomes/centromeres cannot attach (to spindle)
/Chromosomes cannot line up (on spindle) - (So, no) metaphase
A student prepared a stained squash of cells from the tip of an onion root and observed it using an optical microscope. Explain why the student:
1. Used only the first 5 mm from the tip of an onion root.
2. Pressed down firmly on the cover slip. (2)
- Where dividing cells are found / mitosis occurs
- No dividing cells / mitosis in tissue further away / more than 5 mm from tip
- To get (soft) tissue that will squash
- Length that will fit under cover slip
- Single / thin layer of cells / spread out cells so light passes through (making cells / nuclei visible)
Describe and explain what the student should have done when counting cells to make sure that the mitotic index he obtained for this root tip was accurate. (2)
- Examine large number of fields of view / many cells
- To ensure representative sample
or - Repeat count
- To ensure figures are correct
or - Method to deal with part cells shown at edge /count only whole cells
- To standardise counting
A scientist treated growing tips of onion roots with a chemical that stops roots growing at the anaphase stage. Suggest how the chemical stops the growth of roots. (3)
- Stops anaphase / cell division / mitosis
‘accept prevents telophase / cytokinesis’ - (By) stopping / disrupting / spindle fibres forming / attaching / pulling
- Preventing separation of (sister) chromatids
- (So) no new cells added (to root tip)
When preparing the cells for observation the scientist placed them in a solution that had a slightly higher (less negative) water potential than the cytoplasm. This did not cause the cells to burst but moved the chromosomes further apart in order to reduce the overlapping of the chromosomes when observed with an optical microscope. Suggest how this procedure moved the chromosomes apart. (2)
- Water moves into the cells/cytoplasm by osmosis
- Cell/cytoplasm gets bigger
The dark stain used on the chromosomes binds more to some areas of the chromosomes than others, giving the chromosomes a striped appearance. Suggest one way the structure of the chromosome could differ along its length to result in the stain binding more in some areas. (1)
- Differences in base sequences
- Differences in histones/interaction with histones
- Differences in condensation/(super)coiling
What is a homologous pair of chromosomes? (1)
- (Two chromosomes that) carry the same genes
Describe two aseptic techniques she would have used when transferring a sample of broth culture on to an agar plate. Explain why each was important. (4)
- Keep lid on Petri dish / Open lid of Petri dish as little as possible.
- To prevent unwanted bacteria contaminating the dish
or - Wear gloves / Wear mask / Wash hands
- To prevent contamination from bacteria on hands / mouth / Prevent spread of bacteria outside the lab
or - Use sterile pipette /Flame the loop / Flame the neck of the container of the culture
- To maintain a pure culture of bacteria
Describe the appearance and behaviour of chromosomes during mitosis. (5)
(During prophase)
- Chromosomes coil / condense / shorten / thicken / become visible
- (Chromosomes) appear as (two sister) chromatids joined at the centromere
(During metaphase)
- Chromosomes line up on the equator / centre of the cell
- (Chromosomes) attached to spindle fibres
- By their centromere
(During anaphase)
- The centromere splits / divides
- (Sister) chromatids / chromosomes are pulled to opposite poles / ends of the cell / separate
(During telophase)
- Chromatids / chromosomes uncoil / unwind / become longer / thinner
