Cell Division, Cell Diversity and Cellular Organisation Flashcards
Cell cycle
Before dividing, cells must grow and synthesise new organelles and molecules - these processes occur in a particular order, which is known as the cell cycle
Which part of the cell cycle does the most growth and synthesis occur?
Interphase
What percentage of the cell cycle does mitosis typically occupy?
5-10%
What happens at G1?
First growth phase - organelles are synthesised and bio chemicals are produced
What happens during the S phase?
DNA is replicated in the nucleus
What happens during G2?
Second growth phase - energy stores are increased and duplicated DNA is checked for errors
What happens during G0?
- Some mature, differentiated cells such as neurones, no longer divide
- The cycle can be halted in cells with damaged DNA
- These cells are said to have entered G0
Checkpoints of the cell cycle
- The cycle has checkpoints at the end of G1, S, and G2 that verify whether each phase of the cycle has been completed correctly
- These checkpoints are controlled by proteins called cyclins and cyclin-dependent kinases
- The cell cycle can be halted when errors are detected at a checkpoint
The importance of mitosis - processes
Growth, repair (of damaged cells), replacement (of cells, such as red blood cells, that have limited lifespans), asexual reproduction (in eukaryotes)
Chromatids
Two identical DNA molecules (a chromosome) held together at a centromere)
Prophase in mitosis
- Chromatin (uncondensed DNA in a complex with histones) condenses - chromosomes become visible
- Nucleolus disappears
- Centrioles move to the poles of the cell
- Nuclear envelope breaks down (towards the end of prophase)
Metaphase in mitosis
- Spindle fibres (organised by the centrioles) attach to centromeres (towards the end of prophase/beginning of metaphase)
- Chromosomes line up along the centre (equator) of the cell
Anaphase in mitosis
- Spindle fibres shorten
- Centromeres divide
- Chromatids are separated and pulled to opposite poles of the cells
Telophase in mitosis
- Chromatids are at the poles of the cells (and can be referred to as daughter chromosomes)
- Nuclear envelopes reform around each set of chromosomes
- Chromosomes uncoil (and are no longer visible)
- Cell division (cytokinesis) begins
Cytokinesis
- The division of a cell (cytokinesis) begins in telophase, resulting in two genetically identical cells
- Each cell receives approximately half of the organelles and cytoplasm from the original cell
- In animals: a cleavage furrow forms (i.e. cell surface membranes are pulled together by the cytoskeleton)
- In plants: the cell wall prevents cleavage furrows. The two daughter cells are instead separated by new cell wall production down the centre of the original cell
Meiosis
Meiosis is a type of cell division that reduces the number of chromosomes in the parent cell by half and produces four gamete cells
Prophase I of mitosis
- The key events of mitotic prophase occur (i.e. nuclear envelope disintegrates, nucleolus disappears, spindles form, chromosomes condense)
- Homologous chromosomes pair up to form bivalents
- Crossing over occurs
Metaphase I of mitosis
- Homologous pairs (bivalents) line up at the cell equator
- Independent assortment of chromosomes
Anaphase I
- Homologous chromosome pairs are separated (random segregation) - sister chromatids both remain attached to centromeres
Telophase I
- Chromosomes assemble at either pole
- Cytokinesis (cell division) - two haploid cells formed
Prophase II
Chromosomes condense, nuclear envelope breaks down, and spindles form
Metaphase II
- Chromosomes line up at equator (as in mitosis)
- Independent assortment of chromatids
Anaphase II
Random segregation of chromatids
Telophase II
Chromatids assemble at poles
- Cytokinesis (cell division) - four haploid daughter cells are formed
What is crossing over?
- Non-sister chromatids (chromatids from different chromosomes in a homologous pair) interweave (at points called chiasmata) forming bivalents
- Genetic material is exchanged between homologous chromosomes, produces new combinations of alleles
- Happens during prophase I
Independent assortment of chromosomes
- When homologous chromosomes move to the cell equator, the alignment of each chromosome (i.e. on which side of the cell it is positioned) is random
- When homologous chromosomes are separated in anaphase I, many different chromosomes combinations can be formed in daughter cells
- Happens during metaphase I
Independent assortment of chromatids
- Chromosomes line up at the cell equator. The side on which each sister chromatid is positioned is random
- Sister chromatids are no longer identical (due to crossing over), therefore many chromatid combinations are possible in daughter cells
- Happens during metaphase II
Red blood cells/erythrocytes
- Flattened, biconcave shape - increases the SA:V ratio to increase the rate of oxygen diffusion
- No organelles - more space available for haemoglobin
- Flexible due to protein arrangements in membranes - ability to squeeze through capillaries
Neutrophils
- A type of white blood cell
- Multi-lobed nucleus - ability to squeeze through gaps in capillary walls to reach infections
- Many lysosomes - they contain hydrolytic enzymes (to destroy pathogens)
Sperm cells
- Flagellum - for movement towards the egg
- Many mitochondria - to supply energy for movement
- Acrosome - contains digestive enzymes to enable the sperm to penetrate the egg
Palisade cells
- Many chloroplasts (packed close together) - high rate of light absorption for photosynthesis
- Thin cells walls - greater carbon dioxide diffusion rate
- Large vacuole - To maintain turgor pressure
Root hair cells
- Long, narrow extensions of the cell - large surface area to increase water and mineral uptake from the soil
- Large vacuole with a high concentration of dissolved solutes - lower water potential to increase rate of water uptake from the soil
Guard cells
Two kidney-shaped cells with thickened inner cell walls - they control when stomata open (depending on the requirement for gas exchange and water levels in the plant)
Squamous epithelium
- Location:
- Structural features:
- Function:
Ciliated epithelium
- Location: trachea
- Structural features: cilia on the outside of cells
- Function: sweep mucus from trachea
Cartilage
- Location: joints (between bones)
- Structural features: firm and flexible
- Function: protective connective tissue
Skeletal muscle
- Location: attached to bones
- Structural features: contractile proteins
- Function: movement of the skeleton
Xylem
- Location: plant stem
- Structural features: elongated dead cells strengthened by lignin
- Function: transport of water
Phloem
- Location: plant stem
- Structural features: perforated walls
- Function: transport of nutrients
Organs
Organs are collectives of several tissues that combine to perform a function or range of functions, for example, the heart comprises squamous epithelium, endothelium, and cardiac muscle, as well as other tissues
Organ systems
- Organs that work in conjunction with each other
- e.g. digestive system - oesophagus, stomach, liver, pancreas, gall bladder, gastrointestinal tract
Organisational hierarchy
Specialised cells → tissues → organs → organ systems → whole organism
Stem cells
Undifferentiated cells with the potential to differentiate into a variety of the specialised cell types of the organism
Totipotent (potency of stem cell)
- Which cells can they divide to form? - any (and they have the potential to form a whole organism
- Examples: the first 16 cells of an animal zygote, plant meristem cells (including cambium tissue, which differentiates into xylem and phloem tissue)
Pluripotent (potency of stem cell)
- Which cells can they divide to form? - All tissues (but not a whole organism)
- Examples: early embryonic cells (in the blastocyst)
Multipotent (potency of stem cell)
- Which cells can they divide to form? - a limited range of cells
- Examples: haematopoietic cells (in bone marrow), which can differentiate to form all blood cells, including erythrocytes and neutrophils
Current uses of stem cells:
- Drug testing in vitro (i.e. testing drugs on cultured cells in a laboratory)
- Studying developmental biology and disease development in vitro
- Treatment of burns
- Bone marrow transplants to replace stem cells destroyed during cancer treatment
