The cell cycle, mitosis, meiosis Flashcards
What is the cell cycle?
the sequence of events that takes place in a cell
What are the two main phases of the cell cycle?
Interphase
Mitotic phase
What does a cell do during interphase?
cells carry out their major functions
- protein synthesis
- normal metabolic processes
- DNA is replicated and checked for errors
how is the cell cycle regulated ?
- checkpoints
- G1/S - restriction point
-G2/M - other checkpoints eg one in early g1 and halfway through mitosis
- purpose :
. to prevent uncontrolled division that would lead to tumours
. to detect and repair damage to DNA
. because in a certain order The cycle cannot be reversed
. the Dna is only duplicated once during each cycle
What are the 3 stages of interphase?
G1
S
G2
What occurs in G1 of interphase?
- The cell increases in size
- organelles are duplicated
What occurs in G2 of interphase?
- the cell continues growth
- cell double checks duplicated chromosomes for error and makes any necessary repairs
What occurs in S of interphase?
Chromosomes are duplicated
What is G0 of the cell cycle?
Cells leave the cell cycle
Why might a cell leave the cell cycle?
- age
- terminal differentiation
- damage
What occurs in the G1 checkpoint?
- Checks that chemicals needed for replication are present
- checks for any damage to DNA before S phase
What occurs in the G2 checkpoint?
Checks for any errors in the DNA and repairs mistakes
What occurs in the spindle assembly checkpoint?
checks if spindle fibres are connected to chromosomes
What are the two parts to the mitotic stage of the cell cycle?
Mitosis
Cytokinesis
What are the two types of cell division?
Mitosis and Meiosis
When does meiosis occur?
The production of gametes
What occurs in mitosis?
The nucleus divides
What occurs in cytokinesis?
The cytoplasm divides and the two cells are produced
Why is mitosis important?
It ensures the two daughter cells produced are genetically identical
Define chromatid
Two identical copies of DNA (a chromosome) held together at a centromere
Define chromatin
uncondensed DNA in a complex with histones
Define chromosomes
structures of condensed and coiled DNA in the form of chromatin
Define sister chromatid
two identical strands joined by a common centromere
Define centromere
region at which two chromatids are held together
Define centrioles
component of the cytoskeleton made of microtubles, involved in the development of spindle fibres
Define spindle fibres
a protein structure that moves chromosomes
Define homologous pairs
- a pair of chromosomes, one maternal and one paternal, -containing the same genes on the same loci
- which do not necessarily have the same alleles
How is DNA packaged?
- wrapped around histones
- chromatin
- coiled around more proteins
- chromosome
When is DNA found in chromosomes?
only for cell division
Why does DNA form chromosomes for cell division?
Helps protect DNA from damage when it is being moved
Why does DNA wrap around histones?
Makes it more compact
Makes it easy to access
What are the 4 stages of mitosis in order?
Prophase
Metaphase
Anaphase
Telophase
What happens in prophase of mitosis?
- chromosomes condense/become visible
- nuclear membrane begins to break down
- nucleolus disappears
- microtubles form spindle fibres
What happens in metaphase of mitosis?
- chromosomes are moved by spindle fibres
- chromosomes line up along the equatorial plate of the cell
What happens in anaphase of mitosis?
- sister chromatids are pulled apart by spindle fibres
- move towards the poles of the cell
- centromere/chromosomes split
What happens in telophase of mitosis?
- chromatids now called chromosomes
- chromosomes unwinds and become indistinct
- nuclear membrane reforms
Why are centrioles important in cell division?
They produce spindle fibres
Spindle fibres are responsible for the movement of chromosomes within the cell
What happens in cytokinesis of animal cells?
- a cleavage furrow forms
- microfilaments form a ring around the edge of the plasma membrane
- pulls the membrane inwards
- the plasma membrane fuses in the middle to form two cells
What happens in the cytokinesis of plants?
- vesicles carry cell membrane and cell wall components assemble along the plane of division
- the cell membrane first forms as vesicles fuse
- cell wall forms alongside
What do single celled organisms use mitosis for?
-to reproduce
What are 4 functions of mitosis?
- growth of an organism
- repair/replacement of tissues
- asexual reproduction
- development of embryos
How do yeast undergo mitosis?
- yeast reproduces asexually through mitosis
- a bud forms before DNA replicates
- one large and one small cell are produced
What is mitosis in bacteria called?
Binary fission
Define diploid
Normal chromosome number, two chromosomes of each type inherited from each parent
Define haploid
Half the normal number of chromosomes, one chromosome of each type
Define gametes
Haploid sex cell produced by meiosis in organisms that reproduce sexually
Define meiosis
Form of cell division where the nucleus divides twice resulting in a halving of chromosome number and producing 4 haploid cells
Define reduction division
cell division resulting in the production of haploid cells from a diploid cell
Define zygote
initial diploid cell formed when 2 gametes are joined by means of sexual reproduction
How many rounds of cell division occur in meiosis?
2
What occurs in prophase 1 of meiosis?
- nuclear membrane disintegrates
- chromosomes condense
- bivalents are formed
- crossing over occurs at chiasmata
What occurs in metaphase 1 of meiosis?
-homologous pairs of chromosomes line up along the metaphase plate
What occurs in anaphase 1 of meiosis?
- spindle fibres pull homologous chromosomes to opposite poles
- random assortment , pairs move apart
- chromosomes move to the poles
What occurs in telophase 1 of meiosis?
- 2 cells with unidentical chromatids are formed
- nuclear envelope reforms
- chromosomes unwind
What occurs in prophase 2 of meiosis?
- chromosomes recondense, still consist of 2 chromatids
- nuclear membrane disintegrates
What occurs in metaphase 2 of meiosis?
- pairs of CHROMATIDS line up along the equatorial plate
- independent assortment of CHROMATIDS
What occurs in anaphase 2 of meiosis?
-CHROMATIDS pulled apart by spindle fibres
What occurs in telophase 2 of meiosis?
- 4 haploid daughter cells are produced
- nuclear membrane reforms
- Chromatids uncoil
How does independent assortment occur?
The arrangement of bivalents
How does independent assortment lead to variation?
Different alleles can face the poles e.g. the maternal or paternal chromosomes can end up at either pole
Define bivalent
Homologous chromosomes associated in pairs
Define chiasmata
sections of DNA which become entangled during crossing over, break and rejoin
At what stage in meiosis are chiasmata formed?
Anaphase 1
Define recombiant chromatids
Chromatids with a combination of DNA from both homologous chromosomes formed by crossing ver and chiasmata
What two processes occur in meiosis to introduce genetic variation?
Crossing over
Independent assortment
What is crossing over?
Genes are exchanged as sections of DNA on sister chromatids break off and rejoin
How does crossing over lead to genetic variation?
The genes exchanged can be different alleles so the recombiant chromosomes have different sets of alleles to the original chromatids
Besides meiosis, how can variation be introduced? (2 ways)
Random fertilisation
Gene mutation
How does random fertilisation introduce variation?
-each gamete has a unique combination of genes and any of the numerous male gametes can fertilise the egg
When does random fertilisation occur?
When two gametes fuse to form a zygote
How does gene mutation introduce variation?
-bases within a gene can be altered during DNA replication
Can mutation in the germline be inherited?
yes
Can mutation in somatic cells be inherited?
no
Why are different alleles within a population important?
- they introduce genetic variation
- some combinations can provide an organism with an advantageous characteristic e.g resistance to a disease
- so if the environment alters organisms can survive
Why is meiosis important in organisms which reproduce sexually?
- makes haploid cells
- ensures diploid number of chromosomes is restored in fertilisation
- variation generated through crossing over and independent assortment
- allows for a genetically different zygote to be formed through the fusion of gametes
Give 5 differences between mitosis and meiosis
- mitosis has 1 cell division but meiosis has 2
- mitosis maintains chromosome number whilst meiosis halves chromosome number
- mitosis produces 2 daughter cells whilst meiosis produces 4
- daughter cells are genetically identical to the parent cell in mitosis but genetically different in meisosis
- mitosis produces somatic cells whilst meiosis produces gametes
Compare the genetic make up of daughter cells produced by mitosis with the original parent cell
- genetically identical
- have same number of chromosomes
- have the same genes as the parent cell
Why is mitosis important in the life cycle of a human?
- it allows for the growth of a human
- repairs tissues
- genetically identical cells produced
- throughout life maintain chromosome number
Why is mitosis important in the repair of tissues?
- cells produced in mitosis are genetically identical
- this allows cells to carry out the same function as the parent cell
- as the cell has the correct information to develop the same type of cell
What is the difference between sister and non-sister chromatids?
they have different alleles
When does independent assortment occur?
Metaphase I and Metaphase II
Why is it important in sexual reproduction that gametes are haploid?
- without gametes less variation
- gametes fuse to form a diploid cell
- prevents doubling number of chromosome number in each successive generation
Why does meiosis need to have twice as many stages as mitosis?
- gametes must be haploid as when they fuse a diploid cell must form
- halve no. chromosomes from 2n to n
- separate homologous pairs of chromosomes
What is the difference between prophase 1 and prophase 2?
- in prophase 1 chromosomes condense into homologous pairs of chromosomes
- in prophase 1 bivalents form
Why is it important that the genetic information is checked at the checkpoints?
- otherwise mutation of a gene could occur
- which would mean a protein necessary for a particular function might not be produced
- a cell could end up with too many chromosomes
Why is replication of DNA before division of the nucleus essential?
- cell produced are genetically identical, both daughter cells receive a full copy of the parent cell’s DNA
- diploid daughter cells are produced
