CELL AND NUCLEAR DIVISION
● Cell division enables continuity of life based on production of cells
● Unicellular organisms: cell division of one cell reproduced an entire organism (asexual
reproduction)
● Multicellular organisms: cell division enables sexually reproducing organisms to grow and
develop from a single cell (fertilised egg/zygote); continues to function in renewal and repair
after an organism is fully grown
● Cell division consists of:
○ Nuclear division (process of separating nuclear DNA to daughter cells); mitosis
(production of two identical cells) and meiosis (production of gametes)
○ Cytoplasmic division/cytokinesis (process of separating cytoplasm and organelles to
daughter cells
Homologous chromosomes
● In diploid organisms, homologous chromosomes are structurally similar (similar size, shape,
centromere position, sequence of gene loci) but not genetically identical (different alleles at
same loci)
● Allele: alternative form of a gene
Sister chromatids
● Replicated forms of a single chromosome joined together by the centromere
● Separated during anaphase of mitosis and anaphase II of meiosis II
● Structurally identical (similar size, shape, centromere position, number of genes, sequence of
gene loci) -> semi-conservative DNA replication
● Genetically identical/non-genetically identical (after crossing over in prophase I of meiosis I)
Centromere
● Specialised region of a chromosome where two sister chromatids are joined after
semi-conservative DNA replication
● Consists of a short sequence of DNA found in heterochromatin, repeated several thousand
times in tandem and never transcribed
● Associated with protein structures known as kinetochores for attachment of spindle fibres
● Last location to separate during cell division
● Functions:
○ Sister chromatid adhesion
Region of a linear chromosome where the two sister chromatids join
○ Kinetochore formation
Site of assembly of kinetochore, a protein complex that attaches to microtubules of
spindle fibres -> sister chromatids in turn joined via centromere to kinetochore
microtubules
○ Proper chromosome segregation
Only one centromere on each chromosome -> essential for correct segregation of
Copyright © 2019 tonyndr
● Cell division enables continuity of life based on production of cells
● Unicellular organisms: cell division of one cell reproduced an entire organism (asexual
reproduction)
● Multicellular organisms: cell division enables sexually reproducing organisms to grow and
develop from a single cell (fertilised egg/zygote); continues to function in renewal and repair
after an organism is fully grown
● Cell division consists of:
○ Nuclear division (process of separating nuclear DNA to daughter cells); mitosis
(production of two identical cells) and meiosis (production of gametes)
○ Cytoplasmic division/cytokinesis (process of separating cytoplasm and organelles to
daughter cells
Homologous chromosomes
● In diploid organisms, homologous chromosomes are structurally similar (similar size, shape,
centromere position, sequence of gene loci) but not genetically identical (different alleles at
same loci)
● Allele: alternative form of a gene
Sister chromatids
● Replicated forms of a single chromosome joined together by the centromere
● Separated during anaphase of mitosis and anaphase II of meiosis II
● Structurally identical (similar size, shape, centromere position, number of genes, sequence of
gene loci) -> semi-conservative DNA replication
● Genetically identical/non-genetically identical (after crossing over in prophase I of meiosis I)
Centromere
● Specialised region of a chromosome where two sister chromatids are joined after
semi-conservative DNA replication
● Consists of a short sequence of DNA found in heterochromatin, repeated several thousand
times in tandem and never transcribed
● Associated with protein structures known as kinetochores for attachment of spindle fibres
● Last location to separate during cell division
● Functions:
○ Sister chromatid adhesion
Region of a linear chromosome where the two sister chromatids join
○ Kinetochore formation
Site of assembly of kinetochore, a protein complex that attaches to microtubules of
spindle fibres -> sister chromatids in turn joined via centromere to kinetochore
microtubules
○ Proper chromosome segregation
Only one centromere on each chromosome -> essential for correct segregation of
Copyright © 2019 tonyndr
