Cell Division Flashcards
What are the 3 phases within interphase?
- first growth - the cell grows and proteins from which cell organelles are produced are synthesised
- synthesis - when DNA replicates (not the same as DNA replication - period when this happens)
- second growth - when organelles e.g. mitochondria grow and divide and energy stores are increased, the cell increases further in size
What is interphase?
- the cell is engaged in metabolic activity in preparation for cell division
- chromosomes are not visible under a light microscope
What are the stages of the cell cycle?
- interphase
- nuclear division
- cytokenesis
What is nuclear division?
- division of the nucleus, either by mitosis or meiosis
What is cytokenisis?
- the process by which the whole cell divides, resulting in an even distribution of organelles and cytoplasm into each daughter cell
How long is the cell cycle?
- varies between organisms
- typically a mammalian cell takes 24 hours to complete a cell cycle
- of which 90% is interphase
What is the definition of mitosis?
- the division of the nucleus of a cell that results in each of the 2 daughter cells having an exact copy of the DNA of the parent cell
Are daughter cells genetically identical in mitosis?
- the daughter cells are genetically identical to the parent cells and each other
- may not be identical due to mutations
Describe interphase
- DNA widely dispersed in the nucleus
- each molecule replicated to produce a genetically identical molecule
- metabolic activity to prepare for mitosis (second growth)
Describe prophase
- DNA condenses to form visible chromosomes
- centrioles migrate to opposite poles of the cell and spindle fibres are formed
- the nuclear envelope disintegrates
- chromosomes are randomly arranged
Describe metaphase
- spindle fibres develop fully to form spindle apparatus
- spindle apparatus stretches from pole to pole
- chromosomes line up along the EQUATOR of the cell, attaching to spindle fibres by their CENTROMERES
Describe anaphase
- centromeres divide into 2
- the spindle fibres contract, pulling the chromatids to opposite poles
- mitochondria gather near the spindle to supply the energy needed
- (can now be referred to as chromosomes)
Describe telophase
- chromosomes reach opposite ends of the cell and disperse
- the chromosomes are no longer visible
- nuclear envelope reforms and the spindle disintegrates
- new membranes form around daughter nuclei
Describe cytokenesis
- cell divides into 2, producing 2 genetically identical daughter cells
What is the key difference between telophase and cytokenesis?
- telophase - 2 daughter nuclei
- cytokenesis - 2 daughter cells
What is the definition of meiosis?
- nuclear division that results in the production of 4 daughter cells which all contain half the chromosome number of the parent cell
When does meiosis occur?
- during the formation of gametes in the sex organs
- ovary/testes/stamens/carpel
Why is meiosis necessary?
- in sexual reproduction, 2 gametes fuse to give rise to a new offspring
- require 2 haploid cells to join together to make 1 diploid cell
- to maintain the number of chromosomes in adults, chromosomes need to be halved in meiosis
What does n mean when looking at life cycles?
- n is the number of chromosomes
- 2n is the full set
- meiosis happens when 2n halves to n
Why is meiosis important?
- produces haploid gametes (egg, sperm, pollen and ovule)
- introduces genetic variation
Describe the first meiotic division (meiosis 1)
- homologous chromosomes pair up and their chromatids wrap around each other
- equivalent portions of these chromatids may be exchanged in a process called crossing over
- by the end of this stage, homologous pairs have separated with one chromosome from each pair going at random into one of the 2 daughter cells
- this is known as independent segregation
Describe the second meiotic division (mitosis 2)
- the chromatids move apart
- by the end of meiosis 2, 4 cells have been formed each containing 23 chromatids (now called chromosomes)
How does independent segregation increase genetic diversity?
- during meiosis 1 each chromosome lines up alongside its homologous partner
- they line up randomly
- one of each pair will pass into the daughter cell
- which one goes into which depends on how the homologous pairs line up on the equator/spindle
- since pairs line up at random, combination of chromosomes that go into daughter cells in meiosis1 are random
How does crossing over increase genetic variation?
- during meiosis 1, the homologous pairs of chromosomes associate with each other and line up
- their chromatids wrap around each other crossing over many times
- tensions are created and portions of the chromatids break off, these broken portions recombine with the chromatids of the homologous partner
- homologous pair have effectively swapped portions of their DNA
- new genetic combinations of paternal and maternal alleles
- process is known as recombination and happens as a result of crossing over