Chapter 7 : The Cell Cycle and Cell Growth and Differentiation Flashcards
The Cell Cycle Phases
- Inter-phase G1 - primary growth S - synthesis = genome replicated G2 - secondary growth M - mitosis - Prophase - Metaphase - Anaphase - Telophase C - cytokinesis
Inter-phase
- Lots of activity happening in this phase
- Cells prepare for mitosis
- Protein synthesis
- DNA synthesis
- Replication of other cellular structures
- Longest part of cell cycle
The phase of mitosis in eukaryotic cells
- Division of nucleus into 2 nuclei
- Each nuclei has the same number of chromosomes
- Occurs in all somatic (body) cells
- Each new daughter cell has a nucleus complete set of chromosomes
Phases and what occurs in order
Prophase :
- Separates duplicated genetic material carried in -
- nucleus of a parent cell into 2 identical daughter cells
- Chromosomes coil up
- Nuclear envelope disappears
- Spindle fibres form
Metaphase :
- Chromosomes line up in middle of the cell
- Spindle fibres connect to chromosomes
Anaphase :
- Chromosome copies divide
- Spindle fibres pull chromosomes to opposite poles
Telophase :
- Chromosomes uncoil
- Nuclear envelopes form
- 2 new nuclei are formed
- Spindle fibres disappear
Cytokinesis in eukaryotic cells
Cytokinesis :
- Division of the rest of the cell after nucleus divides
- Cytoplasm in animal cells pinch in
- Cell plate forms in plant cells
- After mitosis and cytokinesis the cell returns to interphase
Binary fission / process
The division of a cell into 2 without mitosis; the process by which a prokaryotic cell splits to form 2 daughter cells.
Similar to mitosis but occurs in prokaryotic cells as there is only one chromosome and no centromere.
Process includes:
- DNA replication
- Chromosome segregation
- Cytokinesis
- Produces 2 daughter cells with identical DNA to parent cell
Occurs in eukaryotic cells but only for producing:
- Mitochondria
- Chloroplasts
Disruption of cell cycle regulation
Apoptosis :
- Programmed cell death
- Normal part of an organism’s function
- Important for development, shaping organs and tissues and removing cells that are old or no longer needed
- Almost all multicellular organisms have cells that are born to die
- Failure in Apoptosis can result in many problems, ranging from development defects to cancer
- Formation of cancer happens when apoptosis is avoided and cells do not die as they should
Example of apoptosis :
- Enables tadpole to lose its tail as it becomes a frog
- Allows human embryo to lose the webbing between its fingers and toes
Cell cycle checkpoints :
G0 - resting state
- Doesn’t divide or preparing for division
- Occurs outside of cell cycle
- Damaged cells are located here
G1 checkpoint : Check for: - Cell size - Nutrients - Growth factors - DNA damage
G2 checkpoint
Check for:
- Cell size
- DNA replication
Mitosis checkpoint - spindle assembly checkpoint
Check for:
- Chromosome attachment to spindle
Action of Mutagens in cell cycle
Chemical mutagens :
- Ability to interfere with DNA
- Interrupts its structure, sequence and replication
Physical mutagens :
- Ultraviolet light, X rays and nuclear radiation
- When UV light is absorbed DNA is damaged
- Mutations in tumor suppressor gene p53 play a critical - role in development of precancerous lesions and have - been implicated in all types of skin cancer
Biological mutagens :
- Viruses - take over cells
- Impair cell cycle regulation - results in cancer
- Cause uncontrolled cell growth
Types of stem cells
Stem cells :
Stem cells are undifferentiated cells that have the potential to replicate and to develop into many different kinds of cell
Pluripotent stem cells :
- Capable of giving rise to most but not all tissues of an organism
Example = the inner cell mass
Multipotent stem cells :
Able to give rise to cells that have a specific function
Example = blood stem cells
Adult stem cells
Multipotent :
- Form cells of different kinds of tissue
Advantage = patient’s own cells could be used for treatment
- Risks are lower because patient’s immune system won’t reject its own cells
Disadvantage = are pre-specialised
- Blood stem cells only make blood
- Brain stem cells only make brain cells
- Can replace dead or damaged cells
- Bone marrow has multipotent stem cells that give rise - to all cells of the blood
Embryonic stem cells
- Cultured from frozen embryos that are obtained from in-vitro fertilisation clinics (IVF clinics)
- Ethical issues related to the use of these cells
Obtaining them requires destruction of an embryo - Governments have strict regulations in place for controlling this type of technology
Advantage = pluripotent - Can become any of the cell types of the body
- More versatile than adult stem cells
Disadvantage = come from embryo that’s not derived from patient’s own cells - Patients immune system may reject them
Sources of stem cells for research and therapy - types
Umbilical cord blood and placental stem cells
- Present in blood in umbilical cord and placenta
- Once baby is born cells can be extracted from
- discarded tissue and used for the benefit of children and adults who suffer from bone marrow or blood diseases
- Stored in case baby requires replacement tissues or organs later in life
- Obtained after baby is born and there’s no harm to mother or baby
- Multipotent
Fission
- Single celled eukaryotic or prokaryotic organisms
- Divide into 2 new organisms
- Each is an exact copy of parent cell
- Nucleus is replicated and cytoplasm splits to form 2 new daughter cells
Budding
- New organism is grown on the outside of the parent organism
- Eventually detaches as its own complete organism
- Hydra, yeast
Vegetable propagation
- Only in plants
- Plants grow from cuttings
- New plant forms through mitosis - genetically identical to parent plant
- Plant grows - cells undergo differentiation and become specialised
- Used for agricultural purposes to replicate qualities in plants
Spore formation
- 0Spores are hardy self contained capsules - single celled structures
- Contains exact copy of DNA of the parent
- Can form an entire new organism in right conditions
- Spores can survive for hundreds of years and still germinate
- Prokaryotes, fungi, plants
Biological advantages of asexual reproduction
- Does not require a mate
- Uses less energy
- Offspring identical to parent
- Many offspring produced
Disadvantages of asexual reproduction in certain situations
- Offspring are genetically identical
- Parental care of offspring is limited or none
- Large numbers of offspring may compete for limited resources
- Lack of genetic diversity
Biological cloning - embryo splitting and nuclear transfer
Embryo splitting
- Egg cells (from donor female) are fertilised with male sperm in tissue culture
- Resulting dividing cells are split and coated to promote division
- Cells form a blastocyst - implanted into surrogate mother
- Offspring are twins but born to different mothers
Nuclear transfer
- Process made famous in 1996 - Dolly the sheep
- Nucleus of adult sheep’s mammary cell taken and inserted into ovum - which has nucleus removed
- Resulting blastocyst transferred to surrogate mother
- Method has very low success rate for live births
Issues of cloning
If we’re using adult cells and putting them into eggs to form blastocyst
- Gamete cells - stem cells in embryo that allows offspring to survive.
- Not successful as cells are different and the adult cells aren’t stem cells
- Can’t use adult cells for cloning but you may be able to use adult stem cells instead
- It is failing because they aren’t using stem cells
Application of cloning - agriculture and horticulture
Agriculture :
- Cloning for plants
- Used to save rare species of plants
Horticulture :
- Common for farm animals for food production
SEXUAL REPRODUCTION
Combining the ova (egg) and sperm (sex cells/germ cells) of genetically different parents parents, the offspring have a unique mix of genes from the parents.
Meiosis
Two-phase type of cellular division in which the chromosome number of a cell is halved to the haploid number; meiosis is the basis of gamete formation.
The way the sperm and ova (egg) are formed is through meiosis.
MEIOSIS I - Prophase 1
Interphase :
Replicating DNA
Prophase I :
- Chromosomes condense
- Nucleolus disappears
- Spindle forms with centrioles (if present) at opposite ends
- Homologous chromosomes lie side by side - synapsis
- Pair of homologous chromosomes - maternal and paternal - called bivalent
- Homologous chromosomes coil around each other immediately
- Move slightly apart but chromatids remain in contact at certain points - chiasmata
- At chiasmata chromatids may break and rejoin sometimes rejoining to other chromatids - crossing over
- Crossing over - results in swapping of parts of DNA along with chromatids
- Sometimes chromatids rejoin back to same chromatids with no crossing over - have same DNA sequences as before
- Offspring with different DNA from that in their parents following crossing over - recombinants
- Crossing over - production of gametes with new parental combinations of DNA and is an important source of genetic variation
Meiosis I - Prophase 1
Interphase
Replicating DNA
Prophase I :
- Chromosomes condense
- Nucleolus disappears
- Spindle forms with centrioles (if present) at opposite ends
- Homologous chromosomes lie side by side - synapsis
- Pair of homologous chromosomes - maternal and paternal - called bivalent
- Homologous chromosomes coil around each other immediately
- Move slightly apart but chromatids remain in contact at certain points - chiasmata
- At chiasmata chromatids may break and rejoin sometimes rejoining to other chromatids - crossing over
- Crossing over - results in swapping of parts of DNA along with chromatids
- Sometimes chromatids rejoin back to same chromatids with no crossing over - have same DNA sequences as before
- Offspring with different DNA from that in their parents following crossing over - recombinants
- Crossing over - production of gametes with new parental combinations of DNA and is an important source of genetic variation
Meiosis I - Metaphase I
- Nuclear envelope breaks down
- Homologous chromosomes move together to equator of spindle
- Spindle fibres attach to centromere of each homologous pair
Meiosis I - Anaphase I
Anaphase I
- Spindle fibres retract towards the poles
Pulling maternal and paternal chromosomes of homologous pairs towards opposite poles of the spindle
- Separation or disjunction of each pair of homologous chromosome occurs independently of other - chromosome pairs
Meiosis II - Interphase II
- Brief interphase occurs at the end of meiosis I
- DNA does not duplicate during this interphase
Meiosis II - Prophase II
Prophase II
New spindle forms at the right angles to the first one
Meiosis II - Metaphase II
Metaphase II
- Chromosomes move to equator of spindle
- Spindle fibres attach to centromere of each chromosome
Meiosis II - Anaphase II
Anaphase II
- Spindle fibres retract towards each pole
- Chromatids separate and move apart from each other
- Chromatids become the chromosome of daughter cells
Meiosis II - Telophase II
Telophase II
- Spindle apparatus disappears
- Chromosome decondense to thread like form
- New nuclear envelopes and nucleoli form
Meiosis II - Telophase II
Telophase II
- Spindle apparatus disappears
- Chromosome de-condense to thread like form
- New nuclear envelopes and nucleoli form
Fertilization
- 2 haploid (n) gametes (from each parent) join to form a diploid (2n) zygote
- All somatic (body) cells are diploid (2n)
Means they have n pairs of homologous chromosomes - one originally derived from each parent - Zygote formed has double the amount of DNA of the gamete
- Meiosis halves the amount of DNA
- Fertilisation restores the amount of DNA to required amount for that species
- Human gametes produced by meiosis contain 23 chromosomes
- Fertilisation restores number of chromosomes to 46 (23+23=46) = chromosome number in somatic cells
Chromosomes and variation
- Number of possible combinations that can occur = 2n (n is the number of haploid chromosomes)
- Humans have 2^23 possible combinations which is almost 10 million
- Homologous chromosomes separate and recombine in first division of meiosis, pieces of chromosomes sometimes break and exchange with their homologous pair during crossing over
- Leads to variations in appearance of offspring
- Fertilisation provides potential for different combinations of characteristics in offspring because of the chance that particular gametes combine
- Even though they produce variations in characteristic in sexually reproducing organisms they don’t make new DNA
- Existing DNA is only reshuffled through different combinations
Biological advantages of sexual reproduction
- Gametes with many different combinations of parental chromosomes are possible
- Offspring are genetically different from parents and from each other
Disadvantages of sexual reproduction
- Energy is required to produce organs for sexual reproduction
- Energy is required to find and attract a male
- Competition for mates
