B3 Cell Division and Sex Determination Flashcards
what are the three stages of cell division
- interphase
- mitosis
- cell cytokinesis
summarise what happens during interphase
the cell grows and DNA is copied
what is interphase
the longest phase of the cell cycle
a period of intense metabolic activity of the nucleus
what three metabolic activities occur during interphase
- The cell grows by - replicating DNA - producing proteins and cytoplasmic organelles.
- Centrioles divide in an animal cell.
- Chromosomes appear as long thin threads called chromatin. Just before the cell divides, each chromatin thread replicates, producing two identical sister chromatids joined at the centromere.
what are the four stages of mitosis
Prophase
Metaphase
Anapahse
Telophase
what stages of mitosis is the longest
prophase
what happens in prophase
• Nuclear envelope disintegrates. Nucleolus disappears.
• Chromatin threads condense, coil
and shorten to become discrete chromosomes observable under a light microscope.
• In animal cells, centriole pairs migrate to opposite poles of the cell.
• Spindle fibres continue to develop.
what happens in metaphase
- Spindle fibres (microtubules) attach to both sides of the centromere of each chromosome.
- Chromosomes migrate and align singly at the metaphase (equatorial) plate.
what is the shortest stage fo mitosis
anaphase
what happens in anaphase
• Centromeres divide and sister chromatids are
separated to become daughter chromosomes.
• Spindle fibres (microtubules) shorten and pull
daughter chromosomes to opposite poles of the
cell.
• Daughter chromosomes move with centromeres
leading, producing a characteristic ‘V’ shape pattern.
what happens in telophase
• Spindle fibres (microtubules) break down / disassemble.
• Nucleolus and nuclear envelope re-form.
• Chromosomes uncoil and decondense to
become thin chromatin threads.
what are the three reasons for which we need mitosis
- Genetic stability
- Growth, repair and regeneration
- Asexual reproduction
how does mitosis aid genetic stability
Mitosis produces two daughter cells with the same number of chromosomes. This ensures the preservation of genetic stability across generations.
how does mitosis aid growth, repair and regeneration
o Growth – increasing cell numbers
o Repair of tissues – damaged cells are replaced by exact copies of the original
o Regeneration – replacement of worn out cells
how does mitosis aid asexual reproduction
Offspring is genetically identical to parent, ensures preservation of favourable traits from generation to generation
in terms of cell, how many cancer cells initially arise
A cell may be stimulated to divide, differentiate or die by signals released by neighbouring cells.
what are the four checkpoints
G0
G1
G2
M
what happens in the G1 checkpoint
assessment of cell growth
checks for growth factors, DNA damage and cell size
apoptosis will occur if DNA is damaged and cannot be repaired
what happens in the G2 checkpoint
assessment of DNA replication
checks if DNA is successfully replicated without damage apoptosis occurs if there is irreparable damage
what happens in the M checkpoint
assessment of mitosis
occurs at metaphase
checks for formation of spindle fibres
mitosis is arrested if spindle fibres are not formed
how does cancer cells escape cell cycle control and what does this result in
Cancer cells have escaped precise cell cycle control due to mutation in cell cycle control genes, such that they proliferate indefinitely.
o loss of function mutation of tumour suppressor gene
o gain in function mutation of proto-oncogene to form an oncogene
what is cancer the result of
Cancer is the result of changes in cells that lead to uncontrolled growth and division.
define meiosis
division that produces daughter cells, known as gametes, in which the chromosome number is halved from diploid to haploid so they have a single set of chromosomes
where does meiosis occur
• occurs only in sex cells in sexually reproducing organisms
what comes before meiosis
• preceded by interphase where DNA replication occurs
in meiosis 1, what happens in prophase 1
• Nucleolus disappears and nuclear membrane disintegrates.
• Chromatin condenses (shortens and thickens) until chromosomes become
discrete.
what two unique events happen in prophase 1 during meiosis
synapsis - homologous chromosomes pair up
crossing over
what is synapsis in prophase 1 of meiosis 1
Synapsis occurs and homologous chromosomes pair up to form a bivalent.
o Homologous chromosomes have the same shape, same genes and same length. One comes from the male parent, the other from the female parent.
what is crossing over
non-sister chromatids of homologous chromosomes undergo exchange of alleles, producing new combinations of alleles along the chromosomes. The point where they cross over is called a chiasma (plural: chiasmata)
what happens in metaphase 1
• Pairs of homologous chromosomes align at the metaphase (equatorial) plate.
• Each chromosome is attached to a spindle fibre.
• Independent assortment of homologous
chromosomes occurs.
what genetic variation step occur during metaphase 1
Independent assortment of homologous
chromosomes occurs.
what happens in anaphase 1
Homologous chromosomes separate and are pulled to opposite poles of the cell as the spindle fibres shorten.
what happens in telophase 1
- Spindle fibres disintegrate.
* Nuclear envelope re-forms around each set of chromosomes.
what happens in cytokinesis 1
- The cytoplasm cleaves into two, producing two daughter cells, each with the haploid number of chromosomes.
- The centrioles divide.
what happens in prophase 2
- The two pairs of centrioles move to opposite poles of the cell.
- Nuclear envelope disappears.
- Spindle fibres appear.
what happens in metaphase 2
- Chromosomes migrate and align singly at the metaphase (equatorial) plate.
- The metaphase plate of meiosis II is perpendicular to that of meiosis I.
what happens in anaphase 2
• Centromeres divide. Sister chromatids separate to form daughter chromosomes, which are pulled to opposite poles of the cell as spindle fibres shorten.
what happens in telophase 2
- Spindle fibres disintegrate.
- Nuclear envelopes re-forms around each nucleus.
- Nucleolus re-forms.
what happens in cytokinesis 2
• Cleavage of cytoplasm results in four daughter cells being produced, each with half the number of chromosomes as the parent cell (haploid).
what is unique about each daughter cell produced in meiosis
• Each daughter cell will be genetically different.
what are the two significant reasons for meiosis
- sexual reproduction (to make haploid gametes)
2. genetic variation
how does meiosis cause sexual reproduction to make haploid gametes
o Meiosis produces haploid (n) gametes, which fuse together during fertilisation to produce diploid (2n) cells.
o When male and female gametes fuse during fertilisation, the diploid number of chromosomes is restored in the zygote.
o This maintains the normal (constant) chromosome number through successive generations.
how does meiosis cause genetic variation
o Crossing over between homologous chromosomes at prophase I
new combinations of alleles
o Independent assortment of homologous chromosomes at metaphase
I (the orientation of each bivalent with respect to the poles is random
and is independent of the orientation of the other bivalents) o Random fertilisation (any sperm fuses with any egg)
what are the differences between asexual reproduction and sexual reproduction
Asexual Reproduction
involves one parent
offspring are genetically identical when no mutations occur
Sexual Reproduction
involves two parents
offspring are genetically different in relation to each other and the parents, leading to (increased) variation
what are sex chromosomes
chromosomes that determine the sex of an organism In most mammals including humans,
what is the difference between a male and female sex chromosomes
- Males have an X chromosome and a Y chromosome (XY)
* Females have two X chromosomes (XX)
during gamete production, what do males produce vs females
- Males will produce two types of sperm, containing either X or Y chromosome
- Female gametes (eggs) contain an X chromosome each
