Cell division Flashcards
define somatic
body cell
what are the phases of mitosis
“interphase: 90% of the cycle
- G1 (“growth phase” 1) - Cells prepare for DNA replication/ protein syntheisis occurs
- S (“synthesis”) - DNA replication occurs
- G2 (“growth phase” 2)- Short gap before mitosis / organelles grow and divide, energy stores increased
Mitosis (relatively short)
- prophase, metaphase, anaphase, telophase
Cytokenesis
- splits”
How does CDK control cell cycle
”- Cyclins are produced during each phase of the cell cycle
- A CDK will bind to the cyclin if present
- If CDK binds to cyclin it is activated.
- It will then activate another target protein
- The activated target protein is essential for the next phase of the cell cycle
- if CDK is not produced it will enter a non-dividing phase known as G0”
describe prophase
”- chromosomes condense and become visible
- nuclear envelope breaks down and nucleolus disappears
- centrioles move to opposite poles”
describe metaphase
”- centrioles produce spindle fibers
- SF attached to centromeres of sister chromatids
- sister chromatids line up at the equator”
describe anaphase
”- spindle fibres shorten so sister chromatoids are pulled apart
- centromeres split
- chromosomes move to opposite poles”
describe telophase
”- chromosomes decondensed
- nuclear envelope reforms
- the spindle is broken down”
describe asexual reproduction- binary fission
“1. circular DNA replicates
2. cytoplasmic volume increases - cell elongates
3. Circular DNA moves to opposite poles
4. cell wall starts to form in the middle (septum)
5. septum splits down the middle”
describe the formula for mitotic index
mitotic index = number of cells in mitosis/ total number of cells counted
define meiosis
A type of nuclear division resulting in the formation of cells containing half the number of chromosomes of the parent cells
define loci
The position of a gene on a chromosome
define Chromosome
A linear DNA molecule wrapped around histone proteins in the nucleus
define fertilisation
The fusion of male and female gamete nuclei
defien diploid and haploid
Diploid: A cell in which the nucleus has two complete sets of chromosomes represented by the symbol ‘2n’
Haploid: . Having only one set of chromosomes – represented by the symbol ‘n’
what is a zygote
A cell produced after fertilisation of two gametes during sexual reproduction
What is a homologous chromosome
Matching chromosomes containing the same genes at the same places (loci). They may contain different alleles.
why ar haploids cells essential in meiosis
- If a life cycle involves sexual reproduction then it is necessary for the chromosomes to be halved at some point
- If this did not occur the amount of genetic material would double with each new generation!
what are teh purposes of meiosis I and II
- Meiosis I is known as the reduction division, during which homologous chromosomes separate resulting in haploid cells with replicated chromosomes.
- In meiosis II both of the haploid nuclei divide again, sister chromatids separate as in mitosis resulting in 4 haploid daughter cells
Describe prophase I
- Chromosomes begin to condense
- Homologous chromosomes pair up, this process is known as synapsis, pairs are called - Homologous pairs or bivalents.
- The crossing over of chromatids may occur leading to genetic variation
- Centrioles move to opposite ends of the nucleus
- Nuclear envelope begins to breakdown
- Nucleolus disappears
- The spindle begins to form
how does crossing over lead to variation
- bivalent forms
- chiasmata forms
- equal lengths of allele are exvchanged
- forms recombinant chromosomes
describe metaphase I
- The bivalents line up across the equator of the spindle attached by the kinetochore of the centromere
- random assortment of how bivalents line up creates genetic variation in daughter cell
- Both chromatids from one homologue are attached to kinetochore microtubules from the same pole.
- The chromatids from the other homologue are attached to kinetochore microtubules from the opposite pole
describe anaphase I
- Centromeres do not split (unlike mitosis).
- The homologues (chromosomes) separate and move to opposite poles
- The separate chromosomes move along the spindle microtubules; the centromere first
describe telophase I
- The chromosomes reach the poles of the spindle
- In some cells the nuclear envelope and nucleolus reforms
- The spindle disappears
- Cytokinesis occurs
- Each new cell has a complete set of haploid chromosomes, but each chromosome consists of two chromatids
Describe prophase II
- The nuclear envelope breaks down
- The nucleolus disappears
- The spindle apparatus forms
- The centrioles replicate and move to opposite poles of the spindle
- The chromatids begin to move towards the equator of the spindle
describe metaphase II
- The chromosomes line up across the equator of the spindle as in mitosis
- However, the two sister chromatids of each chromosome are not identical due to crossing over that occurred in prophase I
- The kinetochores of sister chromatids are attached to microtubules from opposite poles of the spindle
- Sister chromatids are attached to microtubules from opposite poles at the centromere via kinetochores
describe anaphase II
- Chromatids separate and are pulled to opposite poles, centromere first, along the spindle microtubules
describe telophase II
- The chromatids reach the poles of the spindle and are now the new chromosomes
- 4 new daughter nuclei begin to form
- The spindle disappears
- Nuclear envelopes begin to reform
- Nucleoli become visible
- The chromosomes become less condensed
- Meiosis is now complete, having produced 4 genetically unique daughter cells each with a haploid nucleus
How does variation occur in meioisis
- Random mutations of an organism’s DNA
- Crossing over:
- Occurs during prophase I, produces recombinant chromosomes which contain DNA from both parents. These chromosomes are therefore different from any of the maternal or paternal chromosomes. - Independent assortment of chromosomes/chromatids:
- Occurs during metaphase I when homologous pairs are aligned randomly at the equator of the spindles
During metaphase II due to random orientation of chromatids. - Random fertilisation:
Explain the difference between meiosis in males and females
In males you produce 4 daughter cells that will all become sperm cells. In females only one of the 4 becomes an egg cell, the other three become polar bodies that eventually die.
