chapter 16 cell division Flashcards
what is mitosis
form of nuclear division that produces daughter nuclei containing the same number of chromosomes as the parent cells = daughter cells are genetically identical to parent cells
– only occurs in cells with nuclei, but can be any cell
what occurs in interphase
→ cells carry out activities to prepare for mitosis - absorbing nutrients, building up protoplasm
→ chromosomes appear as chromatin (long thin threads)
→ centrioles divide in animal cells
→ DNA replication occurs: each chromatin thread replicates to form 2 identical chromatin threads - joined at centromere = sister chromatids → to ensure the chromosome number in parent cell is same as in the daughter cells
Eg 4 chromatin in parent cell = 4 pairs of sister chromatids (number of centromere = number of chromosomes)
what occurs in prophase
in picture: clump of cells
chromatin threads condense, coil and shorten (CCR) = become chromosome (1 pair of chromatin thread = 1 chromosome)
→ by end of prophase:
- nucleolus and nuclear envelope disappear
- 2 pairs of centrioles move to opposite ends of the cell
- asters form around centrioles
- Spindle forms with spindle fibres extending from 1 pole of the cell to the other
what occurs in metaphase
chromosomes line up along equatorial plane of spindle - centromere of each chromosome is attached on both sides to a spindle fibre
what occurs in anaphase
→ centromere divides (chromosomes are no longer connected = form chromatids) - spindle fibres shorten -> pull chromatids apart to opposite poles of cell
→ separated chromatids = form daughter chromosomes (eg 4 chromosomes = 8 chromatids with one centromere each = 8 daughter chromosomes)
what occurs in telophase
- Spindle fibres break down
- Nuclear envelope forms around daughter chromosomes at each pole = forms 1 nucleus at each pole (each nucleus has equal number of chromosomes)
- Nucleolus reforms in each nucleus
- Daughter chromosomes uncoil and lengthen = form thin chromatin threads (1 daughter chromosome = 1 chromatin thread)
what happens in cytokinesis
→ cleavage/furrows appear in cytoplasm between the 2 nuclei
→ furrows deepen = the 2 nuclei separate - produce 2 identical daughter cells
why is mitosis important
Mitosis enables growth of organism
→ mitosis produces new genetically identical cells for the organism to grow
Mitosis is needed for repair of worn out parts of the body
→ produces new cells that replace damaged cells
Mitosis allows asexual reproduction in plants to occur
→ grow daughter cells that are genetically identical to the parent cell
why must genetically identical daughter cells be produced
→ ensure that all daughter cells are genetically stable (genetically identical to parent cell) = all DNA is replicated for subsequent cell division/differentiation = future daughter cells are genetically identical
→ if error in DNA replication - new DNA strand formed (replicated) would be different from original = gene mutation in new daughter cell
→ gene mutation can cause abnormal proteins to be formed/uncontrolled division of cells - lead to excess cells that can form tumour (lead to cancer)
what is meiosis
→ a form of nuclear division that produces daughter nuclei containing half the number of chromosomes (haploid = n) as the parent nucleus (diploid = 2n)
→ occurs in gametes only (reproductive cells)
what happens in interphase (meiosis)
→ DNA replication: chromatin threads replicate - form 2 identical sister chromatids - 1 pair of sister chromatids attached at centromere (eg 4 chromatin threads = 8 sister chromatids = 4 pairs of sister chromatins = 8 chromosomes)
– each nucleus has (at least) 1 maternal chromatin and 1 paternal chromatin = after replication, forms 1 pair of maternal sister chromatids and 1 pair of paternal sister chromatids
→ pair of centrioles divide
what happens in prophase l
- each pair of sister chromatids go through CCR = become chromosomes (eg 1 pair of maternal sister chromatids = 1 pair of maternal chromosome)
- synapsis occurs: formation of homologous chromosome by pairing up - 1 paternal and 1 maternal chromosome pair up along whole length
- crossing over: non sister chromatids from each (maternal and paternal) homologous chromosome may break and exchange parts with each other = produces new combinations of genes along the chromosomes
- Homologous chromosomes repel each other
- Asters form around centrioles - centrioles move to opposite ends of cell
- Spindle fibres form
- Nuclear envelope and nucleolus disappear
what happens in metaphase l
→ pairs of homologous chromosomes arrange themselves along equatorial plane - two chromosomes of each pair face opposite poles of the cell
→ centromere of each chromosome is attached to a spindle fibre
what happens in anaphase l
- spindle fibres shorten = homologous chromosomes separate, pulled to opposite poles of cell
- chromosomes in the homologous chromosome do not separate into chromatids = one chromosome separates from the other chromosome (centromeres separate but do not divide - 1 homologous chromosome is made up of 2 chromosomes)
what happens in telophase l
→ nuclear envelope forms around chromosomes at each pole of the cell
what happens in cytokinesis l
→ furrows form in cytoplasm between the 2 cells - furrows deepen = produces 2 daughter cells with haploid number of chromosomes
→ centrioles divide
Meiosis does not stop here - do not take number of chromosomes in daughter cells here as final number
what happens in prophase ll
→ two pairs of centrioles move to opposite poles of cell
→ nuclear envelope disappears, spindle fibres appear
what happens in metaphase ll
→ chromosomes arrange themselves along equatorial plane (chromosomes are in form of 1 chromosome = 1 pair of sister chromatids)
what happens in anaphase ll
→ centromere divides, spindle fibres shorten
→ sister chromatids of each chromosome separate = form daughter chromosomes - pulled to opposite poles of the cell
what happens in telophase ll
spindle fibres disappear
Nuclear envelope forms around daughter chromosomes at each side of the cell
Nucleolus reforms
what happens in cytokinesis ll
→ furrows form in cytoplasm between the 2 daughter cells in each daughter cell - furrows deepen and separate the cells = form 4 daughter cells
–> chromosomes unravel into chromatin = each daughter cell has half the number of chromosomes (chromatin) as parent cell
why is meiosis important
- produces haploid gametes: nucleus of male gamete fuses with nucleus of female gamete during fertilisation = the diploid number of chromosomes is restored in zygote = maintaining correct number of chromosomes in the species
- Results in genetic variations → crossing over of non-sister chromatids in homologous chromosomes and independent assortment of chromosomes = increases chances of survival of species during changes in environment = those that survive will pass on favourable traits to offspring
how can meiosis cause genetic variation
- Crossing over in prophase l only
→ non-sister chromatids (chromatids from different chromosomes) of homologous chromosomes may cross-over in prophase I of meiosis = crossing-over results in new combinations of alleles along the chromosomes = genetic variation in gametes - Independent assortment of homologous chromosomes in metaphase l
→ homologous chromosomes assort themselves randomly along equatorial plane in metaphase l = resultant gametes contain a combination of chromosomes from maternal and paternal
differences between meiosis and mitosis (7)
- Location: Mitosis occurs in normal body cells during growth or repair of body parts but meiosis occurs in gonads during gamete
formation - no. of chromosomes in daughter cells:
Mitosis - daughter cells contain same
number of chromosomes as
parent cell but in meiosis daughter cells contain half the number of chromosomes as parent cell - genetically identical: in mitosis, daughter cells are genetically dentical to parent cell but in meiosis: daughter cells are not genetically
identical to parent cell - nuclear divisions: mitosis Involves one nuclear division but meiosis involves two nuclear division
- no. of daughter cells: mitosis - 2 daughter cells are produced from one parent cell but in meiosis 4 daughter cells are produced from one parent cell
- pairing of homologous chromosomes: Pairing of homologous chromosomes does not occur in mitosis but homologous chromosomes pair at
prophase I of meiosis - crossing over: No crossing over in mitosis but crossing over may occur at prophase I in meiosis
