mitosis

gene mutation

arises from a change in the sequence of nucleotide bases in the DNA of a gene

chromosomal aberration

two major forms - variation in chromosomal structure, and variation in chromosomal number

deletion

removes a chromosomal segment

duplication

repeats a segment

inversion

reverses a segment within a chromosome

translocation

moves a segment from one chromosome to another, non-homologous one

aneuploidy

a genetic disorder where the cell does not have a chromosome number that is a multiple of the haploid number. Chromosomes are present in either extra or fewer copies than the wild type

trisomic

if a chromosome is present in triplicate, aneuploid cell is said to be trisomic e.g. 2n+1

monosomic

if the cell is missing a chromosome, it is said to be monosomic e.g. 2n-1

non-disjunction

homologous chromosomes do not move properly to opp. poles at meiosis I, OR sister chromatids fail to separate properly to opp. poles at meiosis II

Down syndrome (Trisomy 21)

result of an extra chromosome 21 (a total of 3 copies), so each body cell has a total of 47 chromosomes

mitosis

is a form of nuclear division in eukaryotic cells which produces two daughter nuclei containing identical sets of chromosomes as parental cell nucleus

interphase (end of, before mitosis)

DNA is replicated; nucleus is bound by nuclear envelope; nucleolus is present; single microtubule organising center is duplicated to form two centers, each containing a pair of centrioles in animals (centrioles are absent in higher plant cells); organelles are duplicated

prophase (early, mitosis)

spindle fibres begin to form; centrioles move to opposite poles of cell; nucleolus begins to disappear, nuclear envelope begins to disintegrate; chromatin in the process of coiling, plus shortening and thickening into a condensed chromosome

prophase (late, mitosis)

centrioles migrate to opposite poles of cell; nuclear envelope continues to disintegrate; chromosomes are now even more condensed and visible and appear as two sister chromatids joined together at the centromere; spindle fibres extend from each pole toward kinetochores and middle region of cell (metaphase plate); spindle fibres attach to centromeres of chromosomes

metaphase (mitosis)

centrioles located at poles of cell; spindle fibres attached to centromeres of chromosomes, with attachment of 1 centromere to 2 spindle fibres (1 from each pole); chromosomes aligned at metaphase plate/equator, lining up singly in a row

anaphase (mitosis)

centromere of chromosome divides; daughter chromosomes are led towards the poles by their centromeres resulting in characteristic 'V' shape; spindle fibres elongate and slide in opposite direction, elongating the cell; separated sister chromatids, now called daughter chromosomes, are pulled to opposite poles by shortening spindle fibres

telophase (mitosis)

cleavage furrow (in animal cells) or cell plate (in plant cells) starts to form; spindle fibres disintegrate; daughter chromosomes reach poles of cell, and decondense and lengthen into chromatin; nuclear envelope reforms around chromatin at each pole; nucleolus reappears

cytokinesis in animal cells

cell membrane begins to invaginate towards the region previously occupied by metaphase plate/equator, forming a cleavage furrow, which deepens until the parent cell is pinched into two

cytokinesis in plant cells

fluid-filled vesicles (derived from Golgi apparatus) move to the metaphase plate/equator of cell and coalesce to form cell plate; contents are converted to pectin and cellulose, contributing to middle lamella and cell wall matrix; membranes of the vesicles form cell surface membranes of daughter cells; the spreading cell plate eventually fuses with parent cell wall and cell membrane, separating the daughter cells

significance of mitosis (genetic stability)

mitosis produces daughter cells that are genetically identical to their parent cell; mitosis does not introduce genetic variation, thus maintaining genetic stability within the populations of cells derived from the same parental cells

significance of mitosis (growth)

growth is defined as an increase in number of cells or size of cells; the number of cells within the organism increases and the new cells produced are genetically identical to existing cells

significance of mitosis (regeneration and cell replacement)

mitosis ensures that when damaged tissues are repaired, the damaged cells are replaced by cells that are genetically identical to the original cells

significance of mitosis (asexual reproduction)

a type of reproduction where an organism replicates itself without the production of eggs or without fertilisation. Asexual reproduction takes place when a single parent produces offspring genetically identical to itself

checkpoints

certain control points of the cell cycle where stop and go-ahead signals (eg

cancer (uncontrolled cell division)

occurs when dysregulation of checkpoints of cell division occur or cells escape the cell cycle control mechanism. This leads to uncontrolled division of cells

tumour

a mass of cells that can result from uncontrolled cell division; tumour cells are genetically identical and derived from a single, genetically altered cell (i.e. mutant cell)

meiosis

is a form of nuclear division in sexually reproducing organisms that produces four haploid daughter nuclei, each containing half the chromosome number of the parent cell

reduction division

meiosis is also known as this because resultant daughter cells have half as many chromosomes as their parent cells

meiosis I

involves pairing of homologous chromosomes and subsequent separation of homologous chromosomes into 2 daughter cells (which reduces the chromosome number by half)

meiosis II

involves separation of 2 sister chromatids

synapsis

process by which homologous chromosomes (homologue) pair up to form bivalents; this process is independent of spindle fibres

crossing over (event)

occurs between non-sister chromatids of homologous chromosomes

chiasmata

sites where non-sister chromatids of homologous chromosomes break and rejoin with each other

tetrad

during crossing over, bivalents are seen as tetrads (each tetrad = 2 chromosomes each with 2 chromatids)

prophase I (overview)

homologous chromosomes pair up via synapsis to form bivalents; crossing over occurs between non-sister chromatids at chiasmata, creating new combinations of alleles; centrioles migrate to opposite poles; nuclear envelope disintegrates

metaphase I

homologous pair of chromosomes (tetrad) align along metaphase plate/equator; independent assortment of homologous chromosome occurs - arrangement of 1 pair of homologue at metaphase plate is independent of other pairs

anaphase I

homologues separate to opposite poles; sister chromatids remain attached and move together towards the same pole because centromeres have not yet separated/divided; this pair of sister chromatids is considered one chromosome

telophase I

chromosomes, each consisting of 2 sister chromatids, reach opposite poles; each pole has a haploid set of chromosomes (n); nuclear envelope starts to reform; nucleolus reforms

prophase II

chromosomes condense; centrioles duplicate and move to opposite poles; spindle fibres begin to form; nuclear envelope disintegrates forming vesicles, nucleolus disappears

metaphase II

spindle fibers are completely formed; centromere of each chromosome is attached to kinetochore microtubules; kinetochore microtubules align chromosomes at metaphase plate in a single file

anaphase II

centromeres divide and sister chromatids separate; each chromatid is now called a daughter chromosome, pulled by shortening kinetochore microtubules to opposite poles centromeres first (i.e. led by centromeres)

telophase II

chromosomes reach poles where they decondense and become diffuse/indistinct; spindle fibres disintegrate; nuclear envelope reforms around each group of chromosomes, nucleolus reappears in each daughter nucleus

cytokinesis (meiosis)

the cell divides to give a total of 4 daughter cells. Each daughter cell (n) has half the number of chromosomes as the parent cell (2n)

formation of haploid gametes

meiosis produces haploid gametes (egg and sperm) for sexual reproduction. During fertilisation, the haploid nuclei of male and female gametes fuse to produce a zygote with a diploid number of chromosomes, thus the diploid condition is restored

why meiosis is needed (chromosome number)

meiosis ensures chromosome number in each species is kept constant every generation

genetic variation (meiosis, overview)

meiosis allows for new combinations of alleles in gametes which leads to genetic variation. Events in meiosis that create genetic variation are crossing over, and independent assortment of chromosomes

crossing over (as source of variation)

crossing over of segments of non-sister chromatids of homologous chromosomes occurs at prophase I of meiosis I; this leads to new combinations of alleles on chromosomes of gametes

Mendel's Law of Independent Assortment

during metaphase I, arrangement of one pair of homologues at metaphase plate is independent of the arrangement of the other pairs of homologues; during anaphase I, chromosomes of one homologous pair will separate independently of other pairs to form daughter cells; this results in different combinations of maternal and paternal chromosomes in daughter cells

random fusion of gametes

meiosis results in haploid gametes being formed. Random fusion of any gametes produced by male and female parents results in genetic variation

importance of variation

due to genetic variation, individuals in a population will have different characteristics/phenotypes; when environmental conditions change, certain individuals in the population may have favourable characteristics that allow them to survive in the new environment and will be selected for