Cell Division Flashcards
What is the cell cycle?
Sequence of events between one cell division and the next
Outline the stages of the cell cycle
- Interphase is the longest phase
- Interphase includes G1, S and G2
- In G1 and G2 cell performs normal functions, organelles are replicated and checks are
made to ensure replication has occurred correctly - DNA replicates in S phase
- Mitosis is when nucleus divides
- Stages of mitosis - prophase, metaphase, anaphase, telophase
- Cytokinesis occurs - division of cytoplasm
Which phase usually takes the most time in the cell cycle?
Interphase
During which stage does the cell surface area to volume ratio decrease?
Interphase
Describe what happens in G1
G1 phase
- Cell synthesises mRNA and proteins required for DNA replication
- Some organelles copied
- Cell increases in size
Describes what happens in S phase
S phase
- Genetic material replicated (DNA replication)
Describes what happens in G2
G2 phase
- Additional growth and organelle replication
- Cytoskeleton dismantled
- DNA replication checked for errors
Describe what happens during G0
Cell leaves cell cycle due to:
- Differentiation - cell becomes specialised
- DNA damage - cell ‘dies’ (apoptosis)
How is the cell cycle controlled?
Checkpoints at end of each stage of interphase
Describe the checkpoints that occur during the cell cycle
G1 checkpoint
- End of G1 phase
- Check for cell size, nutrients, growth factors, DNA damage
- If anything wrong, cell enters G0
G2 checkpoint
- End of G2 phase
- Check for cell size, correct DNA replication, cell damage
Spindle assembly checkpoint
- During metaphase of mitosis
- Check for chromosome attachment to spindle
Describe the structure of a chromosome
- Made up of two sister chromatids
- Joined at the middle by a centromere
Summarise the stages involved in mitosis
- Step 1: Prophase
- Step 2: Metaphase
- Step 3: Anaphase
- Step 4: Telophase
Describe the stages in prophase
- Chromosomes condense (become visible under light microscope)
- Nuclear envelope breaks down
- Spindle microtubules extend from centrioles at two ends of the cell to its equator
Describe the stages in metaphase
- Sister chromatids of each chromosome attach to the spindle microtubules
- Via the centromere
- Line up at equator of cell
Describe the stages in anaphase
- Centromeres divide
- Chromosomes start to migrate to opposite poles of cell
Describe the stages in telophase
- Chromosomes reach opposite poles of the cell
- Spindle microtubules break down
- Nuclear envelope reforms
Define cytokinesis
Division of cytoplasm to form two daughter cells
Explain how cytokinesis occurs in animal cells
- Microtubules form ring around centre of cell
- Microtubules constrict, forming a cleavage furrow
- Cell eventually pinched in two, forming two genetically identical daughter cells
Explain how cytokinesis occurs in plant cells
- New cell wall produced across the equator of cell from vesicles containing carbohydrates
- Vesicles fuse together to form cell plate, which fuses with existing cell wall
- There is plasma membrane either side
- Cell divides into two to form two genetically identical daughter cells
What is the product of one round of mitosis?
Two genetically identical daughter cells
Why is mitosis necessary in living organisms?
- Growth, replacement and repair of tissues
- Asexual reproduction
Why do prokaryotes not divide by mitosis?
- No nucleus
- Reproduce by binary fission
Define fertilisation
Fusion of a male (sperm) and female (egg) gamete
Define homologous chromosomes
- Pair of chromosomes
- Position of the genes on each chromosome is the same
- The alleles may be different
What is the same in all parts of homologous chromosomes?
Sequence of genes
What is the difference between sister chromatids and chromosomes?
- The DNA molecules formed by replication prior to cell division are called sister chromatids
- After the centromere splits at the start of anaphase, they are individual chromosomes
Describe the function of meiosis
- Type of reductive cell division
- Produces genetically varied gametes with a haploid number of chromosomes (n)
- Maintains chromosome number after sexual reproduction
Summarise the stages involved in meiosis
- Prophase I & II
- Metaphase I & II
- Anaphase I & II
- Telophase I & II
Describe the stages in prophase I
- Chromosomes condense and pair up - the chromosomes in each pair are homologous
- Crossing over between homologous chromosomes occurs before condensation
- Spindle microtubules extend from the two ends of the cell to its equator
- Nuclear envelope breaks down
Describe the stages in metaphase I
- Sister chromatids of each chromosome attach to spindle microtubules via the centromere
- Homologous chromosomes randomly assemble at equator of cell (independent assortment)
Describe the stages in anaphase I
- Homologous chromosomes pulled to opposite poles (this halves the chromosome number)
- Each chromosome still consists of two chromatids
Describe the stages in telophase I
- Sister chromatids arrive at the poles of the cell and decondense
- Nuclear envelope reforms
- After telophase, the cell divides by cytokinesis
Describe the stages in prophase II
- Sister chromatids condense and pair up
- No crossing over
- Spindle microtubules extend from the two ends of the cell to its equator
- Nuclear envelope breaks down
Describe the stages in metaphase II
- Sister chromatids of each chromosome attach to the spindle microtubules via the
centromere - Sister chromatids randomly assemble at the equator of the cell (independent assortment)
Describe the stages in anaphase II
- Centromeres divide
- Chromosomes move to opposite poles
Describe the stages in telophase II
- Chromosomes arrive at the poles of the cell and decondense
- Nuclear envelope reforms
- After telophase, the cell divides by cytokinesis
- Results in four haploid cells
- Each with 23 chromosomes made from a single chromatid
What is the end point of the first division of meiosis?
- Separation of homologous chromosomes
- Creating 2 genetically varied haploid cells
What is the end point of the second division of meiosis?
- Separation of sister chromatids
- Creating 4 genetically varied haploid cells
During which stage of meiosis does crossing over usually occur?
Prophase I
How does meiosis promote variation?
- Crossing over of non-sister chromatids in prophase I
- Independent assortment in metaphase I and II
- Second division of meiosis separates alleles further
- Possible combinations of alleles in gametes is 2n
Describe two ways in which genetic variation is brought about during meiosis
Crossing over during prophase I
- Homologous chromosomes pair up and sections of non-sister chromatids are
swapped
- Produces chromatids with new combinations of alleles
Random orientation of pairs of homologous chromosomes in metaphase I and II
- For each pair of chromosomes there are two possible orientations when aligned at
equator of cell
Different combinations of alleles can be made in each daughter cell
Define bivalent
Pair of homologous chromosomes
Define chiasmata
- Physical connection between non-sister chromatids
- Occurs during prophase I
- Hold homologous chromosomes together as a bivalent until anaphase I
- Can result in the exchange of alleles (crossing over)
Define recombinant chromosomes
Homologous chromosomes that have exchanged alleles by crossing over
Explain how the recombinants are formed during meiosis
- Recombination occurs in prophase I of meiosis
- Homologous chromosomes come together in pairs
- Chiasmata form between non-sister chromatids
- Crossing over takes place
Explain how independent assortment of homologous chromosomes in metaphase I leads to variation
- Bivalents are orientated randomly on the equator during metaphase I of meiosis
- Orientation of one bivalent does not affect orientation of other bivalents
- Pole to which allele on one bivalent moves during anaphase I does not affect the pole to
which alleles on another bivalent move - Independent assortment will not occur if two genes are linked
How are gametes responsible for introducing variation?
- Gametes underpin variation within all species which reproduce sexually
- Fertilisation is a random process
- One gamete is from a male, and one from a female
Define tissue
A group of cells working together to perform a function
Define organ
A group of tissues working together to perform a function
Define organ system
A group of organs working together to perform a function
Define organism
A group of organ systems working together to perform a function
How are specialised tissues produced in multicellular organisms?
- By differentiation
- Some genes in a genome are expressed whilst others are not
How are sperm cells specialised for their function?
- Acrosome - contains digestive enzymes
- Helical mitochondrion - produces ATP needed to help sperm swim
- Microtubules - ensure tail beats side to side to propel sperm forwards
- Protein fibres - strengthen tail
- Haploid nucleus - contains 23 chromosomes in humans
- Whiplash tail (flagellum) - propels sperm up vagina
How are erythrocytes (red blood cells) specialised for their function?
- Flattened, biconcave shape - increases SA:V ratio
- No nuclei and few organelles - increases space for haemoglobin (carries oxygen)
- Flexible - can squeeze through narrow capillaries
How are neutrophils specialised for their function?
- Type of white blood cell
- Multi-lobed nucleus - easier to squeeze through small gaps
- Granular cytoplasm - contains many lysosomes containing hydrolytic enzymes to digest
pathogens
How are squamous epithelial cells specialised for their function?
- Line blood vessels
- Flat, smooth and thin - short diffusion pathway
How are ciliated epithelial cells specialised for their function?
- Line trachea to ‘waft’ mucus up to the throat
- Hair-like structures - collect and move mucus up the trachea
How are palisade cells adapted for their function of absorbing light?
- Lots of chloroplasts - absorb light and carry out
photosynthesis - Rectangular shape - can be packed close
together - Thin walls - increases rate of diffusion of CO2
How are root hair cells adapted for their function of absorbing water and minerals?
Long root hairs - increase surface area,
maximises uptake of water and minerals
How are guard cells adapted for their function of opening and closing the stomata?
- Found in pairs either side of stomata - Become less swollen when lose water - closes
stomata to prevent further water loss
How is cartilage adapted for its function of reducing friction between bones?
Contains fibres of elastin and collagen
How is muscle adapted for its function of moving parts of the body?
Contains myofibrils of actin and myosin - allow muscle to contract and relax
How is epidermis adapted for its function of covering the surface of leaves?
- Covered by waxy cuticle - reduces water loss - Stomata present - controls gas exchange and
water loss
How is xylem tissue adapted for its function of transporting water and ions?
- Cells are dead and hollow - forms tube for water
- Cells strengthened by lignin - provides support for plants
How is phloem tissue adapted for its function of transporting sucrose?
Sieve tube elements have few organelles - more
room for transport of sucrose
Companion cells present to provide key
metabolic products for sieve tube elements Sieve plates are perforated - let sucrose through to the next cell
What are stem cells?
Undifferentiated cells
Define potency
Ability to differentiate into different cell types
Define totipotent
- Stem cells that can differentiate into any type of cell
- Including umbilical cord and amnion
- e.g. a fertilised egg or a zygote
Define pluripotent
Define pluripotent
Stem cells that can differentiate into all tissue types
- e.g. embryo cells
Define multipotent
Stem cells that can differentiate into a range of cells within a specific tissue
- e.g. stem cells in bone marrow differentiate into different blood cells
What are the two sources of animal stem cell?
- Embryonic
- Adult e.g. those found in bone marrow
Why are stem cells useful in repairing diseased or damaged organs?
- Continuously divide and replicate
- Can differentiate into different types of specialised cell
Where are plant stem cells found?
- Meristem tissue
- Present wherever plant is growing e.g. root and shoot tips
- Present in cambium between xylem and phloem
- Produce xylem vessels and sieve tube elements
List some potential treatments involving stem cells
- Heart disease - replace damaged muscle tissue
- Type 1 diabetes - replace damaged insulin-producing cells in pancreas
- Parkinson’s disease - replace damaged cells in the brain
What is developmental biology?
Study of the changes that occur as multicellular organisms grow and develop
What role can stem cells play in developmental biology?
Enable scientists to study differentiation of any type of cell
When compared to other body cells, which characteristic of stem cells is the most important for therapeutic uses?
Enable scientists to study differentiation of any type of cell
When compared to other body cells, which characteristic of stem cells is the most important for therapeutic uses?
Ability to differentiate into different types of specialised cell (potency)
Describe some ethical concerns with using embryonic stem cells
- Embryos count as human life
- Embryos are unable to decide if they want to be used in this way
Provide arguments for the use of embryonic stem cells
- Early embryos are ‘only’ small balls of cells
- Large embryos used by IVF are never implanted - surely better use is to help save lives
- No nervous system - embryos unable to feel pain
Advantages of adult stem cells over embryonic stem cells
- Adults can give consent for use of their stem cells
- No rejection problems
- No death of embryos used to provide stem
cells
