2.1.6: Cell division, cell diversity and organisation Flashcards
What are chromosomes made up of?
Chromatin (DNA and histome proteins)
What is meant by the term ‘homologous pair of chromosomes’?
One member of the pair comes from each parent.
Both chromosomes have some length, pattern of bonding, order, genes but DIFFERENT ALLELES
What is interphase?
The growth period of the cell cycle, between cell divisions (mitotic phase).
What are the stages of interphase?
G₁
S
G₂
What are the three main stages of the cell cycle?
- Interphase
- Mitosis (division of the nucleus)
- Cytokinesis (division of the cytoplasm)
What is G₀?
A phase when the cell leaves the cell cycle.
Can be temporary or permanent.
Why might a cell enter G₀?
- If cell’s DNA becomes damaged (so not viable)
- If the cell undergoes differentiation
- Due to age, when cell has reached maximum number of divisions
What occurs in G₁?
First phase of growth
• New organelles formed
• Intense biochemical activity –> cell increases in size
• Cytoplasm is active
What happens in the S phase of interphase?
• DNA is replicated
chromosomes ⟶ chromatids
What occurs in G₂?
Second phase of growth
• More growth of cell
• Increase in energy stores
• Preparation for mitosis
What does the G₁ checkpoint check for?
- Cell size
- Nutrients
- Growth factors
- DNA damage
What does the G₂ checkpoint check for?
- Cell size
- DNA replication
- DNA damage
What does the spindle assembly checkpoint check for?
Chromosome attachment to spindle
What are the checkpoints in the cell cycle?
- G₁ checkpoint
- G₂ checkpoint
- Spindle assembly checkpoint
What is mitosis?
Cell division of the nucleus which results in genetically identical daughter cells.
Why is mitosis necessary?
- Growth in a multicellular organism
- Repair of tissues by replacement of cells
- Asexual reproduction in eukaryotic single-celled organisms
- Production of B plasma cells in immune response
- production of gametes from haploid cells
- production of new stem cells
Name the four stages of mitosis
1) Prophase
2) Metaphase
3) Anaphase
4) Telophase
Describe the structure of chromosomes following the S phase of interphase
Each chromosome is now made of two chromatids held together at the centromere.
What occurs in mitotic prophase?
- Chromosomes supercoil
- Nuclear envelope breaks down –> fragmented into vesicles
- Centrioles produce spindle fibres from opposite poles
- Spindle fibres attach to centromere
What occurs in mitotic metaphase?
• Chromosomes moved by the spindle fibres to metaphase plate
What occurs in mitotic anaphase?
• Centromeres divide
• Chromatids pulled to opposite ends of cell by contracting spindle fibres
⟶ characteristic V shape
What occurs in mitotic telophase?
• Chromosomes assemble at each pole ⟶ Start to uncoil • Nuclear envelope forms • Nucleolus forms • Cytokinesis begins
Describe cytokinesis in animal cells
1) Cleavage furrow forms
2) Cytoskeleton pulls membrane inwards until the cell separates in two
Describe cytokinesis in plant cells
1) Due to cell wall, cleavage furrow cannot form
∴ Vesicles from Golgi assemble along same plate as metaphase plate was
2) Vesicles fuse with each other and the cell membrane
3) Cell wall forms after this to avoid osmotic lysis
Mitotic index calculation
MI = no. cells with condensed chromosomes
total number of cell
Asexual reproduction
production of genetically identical offspring from one parent.
–> Bacterial asexual reproduction is binary fission, not mitosis.
Allele
Variation of a gene
Meiosis
Division of a cell to form a gamete with a haploid number of chromosomes; reduction division; leads to genetic variation.
Specialised cell
A cell that has differentiated.
Differentiation
The process by which a cell becomes specialised.
Genetic code
- Degenerate
- Triplet code/Consisting of codons (1 codon from each three bases)
- Non-overlapping
- Universal = same 3 codons, same 1 amino acid (any organism)
- More than one codon codes for same amino acid
Transfer RNA (tRNA)
Small RNA molecule in clover leaf shapes
Cells can differentiate by:
- Changing shape
- Number of organelles
- Cell contents (e.g. haemoglobin)
Totipotent stem cells
Cells that can become any cell types; 8-16 cell stage.
Pluripotent stem cells
Form any type of cell but not a whole organism.
Multipotent stem cells
Form a range of cells within a tissue; haematopoietic stem cells in bone marrow.
Stem cells
Undifferentiated cell, not adapted for particular function.
How does meiosis result in genetic variety?
- Independent assortment of maternal and paternal chromosomes. (MI, MII)
- Random segregation of chromatids in metaphase II.
- Crossing over of segments of individual maternal and paternal chromosomes.
Bivalent
Pair of homologous chromosomes
Chiasma
Area where sections of non-sister chromatids cross over.
Recombinant
Result of crossing over between non-sister chromatids
Prophase I
- DNA has been replicated: each chromosome consists of two sister chromatids joined at the centromere
- Homologous pairs come together to form bivalent (maternal and paternal next to each other)
- Crossing over occurs –> alleles exchanged
- Recombinants formed
- (Steps from mitotic prophase)
Metaphase I
- Homologous pairs assemble along metaphase plate
* Alignment is random and independent –> “independent assortment”
Anaphase I
• The two chromosomes from each homologous pair are pulled to opposite poles
Telophase I
- Spindle microtubules break down
- Chromosomes uncoil
- Nuclear envelope forms
- Each cell contains one chromosome from each homologous pair
Prophase II
- Chromosomes supercoil
- Centrioles move to poles and produce spindle fibres
- Nuclear envelope breaks down
Metaphase II
- Individual chromosomes align along metaphase plate
* Random orientation increases genetic variety
Anaphase II
- Centromere splits
* Chromatids pulled to opposite poles of the cell
Telophase II
- Nuclear envelope forms
- Chromosomes uncoil
- Result = genetically different haploid cells develop into gametes
Specialisation of xylem
- Vascular tissue = transport of water and minerals
* Lignin is waterproof and provides structure
Specialisation of phloem
- Transport of organic nutrients (sucrose from leaves, photosynthesis)
- Sieve tube cells, separated by perforated walls called sieve plates
Specialisation of epidermis
- Waxy waterproof covering reduces water loss
* Stomata for CO₂/O₂ control
Specialisation of squamous epithelium
- Squamous cells form “pavement epithelium”
- Flat cells, layer one cell thick
- Forms lining of lungs –> allows rapid diffusion
Specialisation of ciliated epithelium
- Goblet cells release mucus to trap unwanted particles
* Cilia beat rhythmically
Cartilage specialisation
- Contains elastin and collagen
* Chondrocyte cells embedded in extracellular matrix, prevents ends of bones rubbing together and causing damage
Muscle specialisation
• Skeletal muscle fibres contain myofibrils which contain contractile proteins
Erythrocyte specialisation
- No nucleus/organelles
- Biconcave –> higher SA to V ratio
- Flexible
Neutrophil specialisation
- Multi-lobed nucleus –> squeeze through gaps
* Granular cytoplasm contains lysosomes
Sperm cell specialisation
- Male gamete to deliver genetic info to female gamete
- Tail (flagellum) so they are capable of movement
- Many mitochondria to supply energy to swim
- Acrosome contains digestive enzymes to digest protective layers around ovum
Palisade cells
- Rectangular shape for tight packing
- Chloroplasts –> can move w/in cytoplasm to absorb more light
- Thin cell walls to allow fast diffusion
- Large vacuole to maintain turgor pressure
Root hair cells
- Long extensions to increase SA –> maximise water/mineral uptake
- Vacuole w/ cell sap lowers water potential of cell
Guard cells
- Stomata to allow CO₂/O₂ entry/exit
* When water lost, guard cells become less swollen, stomata close
Where are stem cells found in plants?
Meristematic tissue, wherever growth is occurring.
Why is meiosis needed?
To produce haploid cells and genetic variation.
Potential uses of stem cells in medicine
- Repair of damaged tissues
- Treatment of neurological conditions such as Alzheimer’s and Parkinson’s
- Research into developmental biology
