Compare and contrast a TEM microscope with a SEM. (5)
Similarities:
- both use a beam of electrons
- both req. dead specimens
- electrons picked up by an electromagnetic lens
- generates highly detailed image
Differences:
- TEM has higher resolution than SEM
Explain the importance of specialised cells. (2)
- cells differentiate to perform a specific function
- these cells form the basis of tissues, organs & organ systems
Why is mitosis so important? (2)
- for growth
- to replace cells
Describe the appearance and behaviour of chromosomes during mitosis. (6)
- Prophase:
^ chromosomes coil - Metaphase:
^ chromosomes line up at cell centre
^ chromosomes are attached to spindle fibres - Anaphase:
^ chromosomes pulled to opposite ends of cell by spindle fibres - Telophase:
^ chromosomes uncoil
Describe the role of centromere during mitosis. (1)
- the site of attachment for spindle fibres
Explain why it is important for DNA replication to take place before mitosis can occur. (1)
- ensures that each daughter cell receives an exact copy of the DNA
Describe 2 adaptations of an egg cell for its function. (2)
- large in size bcs contains food stores
- zona pellucida forms a barrier after fertilisation
Describe events that occur during fertilisation. (5)
- acrosome releases digestive enzymes to break down zona pellucida
^ acrosome reaction - egg cell releases contents of cortical granules
- zona pellucida rapidly thickens and hardens
^ cortical reaction
Describe the events that occur during meiosis. (6)
crossing over:
- non sister chromatids can cross over and get entangled
- entanglement places stress on DNA molecule
- this creates new allele combinations on chromosomes
Independent assortment:
- orientation of homologous pair is independent
- chromosomes separated and pulled to different poles
- this can create various different allele combinations in gametes
Define: autosomal linkage. (2)
- only occurs on autosomes
- 2 genes on some autosomes do not assort independently
Define: epigenetics. (2)
- inherited changes in DNA
- without changes to DNA’s base sequence
Explain what methylation is and how it makes genes inactive. (4)
- methylation is the addition of methyl groups
- to cytosine
- this changes DNA sequence
- therefore, transcription is inhibited
Explain how stem cells develop into nerve cells. (4)
- response to a stimuli e.g. hormones activates genes within stem cells
- activated genes transcribed
- mRNA translated into proteins
- proteins modify cell
Give 2 ways genetic variation can be measured. (2)
- comparing observable features
- comparing amino acid sequence
