Download Function of Mitosis

Biology Summary
Chapter 5
5.1
Cell cycle is a continuous sequence of cell growth and division
- consists of two main stages :
- Interphase (growth stage)
- Cell makes new molecules
- Cell volume and mass increases
- DNA is copied during DNA replication
- Two stages of interphase:
- G1 (gap 1)
- Rapid growth
- Cells carry out metabolic activities to prepare for next stage
- After G1 phase completed, cell either enter s phase or enter
rest phase
- Cell in rest phase can function but can’t progress through the
entire cell cycle so no new cells produced in this case
- Synthesis (S phase)
- DNA is synthesized and replicated
- G2 (gap 2)
- Centrioles replicate
- Cells prepare to undergo division
- Division stage
- Has two processes:
- Mitosis
- Division of cell’s nucleus
- Cell division
- Division of cytoplasm to form two new cells
-
division are the shortest events in the cycle of a cell
timing of the cell cycle and the lengths of different phases depend on the
type of cell and its environment
through the process of mitosis and cell division, organisms can also
regenerate damaged tissues
division stage is necessary for maintenance of the body
some cells need to be replaced because they cannot function properly or
when they die
mitosis and cell division occur in all somatic (body) cells
new cells that are produced for growth or repair are identical to the
previously existing cells
Chromosomes are structures within the nucleus that hold the genetic
information needed to maintain the cell and to make new copies of the cell
-
each chromosome is made up of two sister chromatids that are held
together by centromere
sister chromatids are genetic copies of each other
Function of Mitosis
- a body cell contains 2 copies of every chromosome
- during cell division, the parent cell divides to produce two new daughter
cells
- mitosis ensures that each daughter cell contains the same number of
chromosomes and the same genetic information as the parent cell
- important because each new cell must have a complete set of genetic
instructions to maintain itself and to produce new cells
Phases of Mitosis
Prophase
- chromosomes have condensed and thickened to form visible
chromatids during late interphase
- at this stage each chromosome is already X-shaped
- each half of X is one copy of the original chromosome
- nuclear membrane and the nucleolus disappear
- centrioles made up of microtubules migrate to opposite poles of the
cell
- spindle fibres (made of microtubules) start to form between the two
centrioles
Metaphase
- spindle fibres attach to the centromere of the chromosomes and pull
them to the middle (equator) of the cell
- a spindle fibre from one pole attaches to the centromere of one
chromatid
- spindle fibres from the opposite pole attach to the other chromatid at
the centromere
Anaphase
- centromere splits apart
- as the spindle fibres shorten, chromatids are pulled to opposite poles
of the cell
Telophase
- chormatids reaches the two opposite poles
- each piece of chromatids is now called a single, non-replicated
chromosome
- chromosomes now begin to unwind and become less visible
Cytokinesis is the separation of the cytoplasm and the formation of two new
daughter cells
- spindle fibres break down and disappear
- nucleolus reappears
- nuclear membrane forms around each new set of chromosomes which are
located at the opposite poles of the cell
- cytoplasm divides between the two halves of the cell
- in plant cells, a cell wall called cell plate forms and separates the two
newly formed nuclei
- in animal cells, the cell membrane pinches in to divide the cells
Error in Mitosis
Mutation is an error in the DNA sequence that can disrupt the mitotic process
- can be caused by various mutagens ie toxic compounds, radiation,
viruses, etc
- when mutation is found in parent cell, it is copied during prophase and is
passed on to the daughter cells
- certain genes work like switches to regulate the rate of mitosis
- of these genes are altered by mutation, the rate of mitosis will be affected
- for example once a cell has completed its cell cycle, certain genes
become activated
- these genes then produce proteins that stop the process of mitosis
- when these genes are inactivated, mitosis can continue and the cell can
divide to produce new cells
- mutation could permanently inactivate these genes so as a result the cells
could begin to divide uncontrollably
Oncogenes are genes that can be activated by a mutation ie retinoblastoma
5.2
Function of Meiosis
- reproduction involves the union of two cells to form zygote that contains
chromosomes from both parents
Meiosis is a special type of cell division that occurs only in reproductive organs
- produces reproductive cells called gametes
gametes = egg / sperm = haploid = n
- gametes contain only one copy of each type of chromosome that the
diploid has
zygote = diploid = 2n
Reduction division is the first part of meiosis that reduces the chromosome
number from diploid to haploid
- each human sperm or egg cell contains 22 autosomes and one sex
chromosome
Autosomes are chromosomes that are not directly involved in determining
the gender of an individual
Sex chromosomes determine the gender of an individual
- possible sex chromosome patterns:
X X : female
X Y: male
Phases of Meiosis
- meiosis follow a sequence of phases that is similar to mitosis but meiosis
involves two sequences of each phase
Interphase
- chromosomes replicate
- after replication each chromosome is made up of a pair of identical sister
chromatids that are joined by a centromere
Meiosis I
Prophase I
- similar chromosomes called homologous chromosomes pair to form
homologous pairs
- the homologous pairs that’s made up of 4 chromatids is called a tetrad
- where does each member of a homologous pair come from?:
- each diploid cell has 2 copies of each chromosome
- one copy from egg, one copy from sperm
- during fertilization the union of gametes forms a diploid zygote
- all cells in the fetus contain copies of chromosomes of this original
diploid zygote
- therefore each cell has one copy of each of mother’s chromosomes
(maternal origin) and one copy of each of father’s chromosomes (paternal
origin)
- homologous chromosomes are similar to each other but not identical
- chromosomes are homologous because they are made up of the same
genes but may have different forms of these genes called alleles
- alleles can be recessive or dominant, which can determine whether a trait
is expressed or not
- when pairing up, crossing over of chromatids can occur, in which nonsister chromatids exchange genes
- this allows for the recombination of genes in each chromosome and allows
genetic variation
- as a result of crossing over, individual chromosomes contain some genes
from maternal origin and some genes from paternal origin
- without crossing over, every chromosome would have only a maternal
origin or a paternal origin
Metaphase I
- a spindle fibre attaches to the centromere of each chromosome
- spindle fibre from each pole attach to one pair of sister chromatids and
pull each tetrad to the equator of the cell
- chromosomes do not line up in single file, but instead they line up in their
homologous pairs
- in each pair, one homologous chromosome is positioned on one side of
the cell’s equator and the other is positioned on the other side of cell’s
equator
- chromosomes that come from one parent are not positioned on the same
side of the cell’s equator
- they are positioned randomly so that some sister chromatids of maternal
origin face one pole while other sister chromatids of maternal origin face
the other pole of the cell
- this random positioning of tetrads along the equator is called independent
assortment
Anaphase I
- homologous chromosomes separate and move to opposite poles of the
cell
- pulled apart by the shortening of spindle fibres
- centromere does not split and the sister chromatids are held together
- so only one chromosome from each pair will move to each pole of the cell
Telophase I
- does not occur in all cells
- if telophase doesn’t occur, cell division goes to meiosis II
- if telophase does occur, homologous chromosomes begin to uncoil and
the spindle fibres disappear
- cytoplasm is divided
- nuclear membrane forms around each group of homologous
chromosomes and two cells are formed
- each new cell contain one copy of each chromosome
- each chromosome already consists of two chromatids, so a second
chromosome replication does not take place between telophase I and
prophase II of meiosis
- in females, meiosis II occurs after the egg is fertilized by a sperm cell
- at the end of telophase I, each cell contains some maternal chromosomes
and some paternal chromosomes due to the independent assortment
- each cell also contains chromosomes that are made up of a combnation of
maternal and paternal alleles
Meiosis II
- the phases of meiosis II are identical to mitosis
- two cells from telophase I go through prophase II, metaphase II, anaphase
II and telophase II
- each cell beginning meiosis II is haploid, but consists of replicated
chromosomes (each consisting of 2 chromatids)
- at the end of meiosis II the daughter cells are still haploid but each cell
contains single unreplicated chromosomes ( no longer made up of two
chromatids attached together )
- the daughter cells at the end of meiosis II are called gametes in and
spores (or gametes) in plants
Gamete Formation
Gametogenesis is the end result of meiosis which is the production of gametes
Spermatogenesis is the process of male gamete production in animals
- takes place in male reproductive organs called testes
- production of sperm starts with a diploid germ cell called
spermatogonium
- this cell enlarges and undergoes meiosis I and meiosis II
- the final product is 4 haploid sperm cells
- each sperm cell has the same number of chromosomes
- after meiosis II the sperm cells develop into mature sperm
- each cell loses cytoplasm and the nucleus forms a head
- long tail-like flagellum is formed for locomotion
- spermatogenesis can occur throughout the year in some organisms
- in other organisms sperm production only occurs during breeding season
Oogenesis is the process of female gamete production in animal
- take place in female reproductive organs called ovaries
- production starts with a diploid germ cell called oogonium
- this cell enlarges and undergoes meiosis I and meiosis II
- at the end of meiosis I the cytoplasm is not equally divided between the
two daughter cells
- the cell that receives most of the cytoplasm is called the primary oocyte
- the other cell is called a polar body and is not a viable sex cell
- as primary oocyte undergoes meiosis II, the cytoplasm is again unequally
divided
- only one cell become an egg (ovum) and contains most of the cytoplasm
- purpose of unequal division of cytoplasm is to provide the ovum wit
hsufficient nutrients to support the developing zygote in the first few days
following fertilization
- meiosis I and meiosis II are not continuous in many organisms
- for example, in humans, meiosis I begins in the ovarian tissue of the
embryo before birth and does not continue beyond prophase I
- continuation of meiosis I occurs after the female reaches puberty
- only one oogonium undergoes this process each month
- meiosis II takes place after fertilization by a sperm cell
- production of ova in females continues until menopause which usually
occurs between 40 and 50 years of age
Meiosis and Genetic Variation
- genetic variation would not be possible without two important processes of
meiosis
1. crossing over between chromosomes during prophase I allows
different combinations of genes
- usually two or three crossovers occur per chromosome
2. variation depends on how each pair of homologous chromosomes
lined up during metaphase I
- when the chromosomes lined up during metaphase I, some
chromosomes of paternal origin move to one pole of the cell while
other chromosomes of paternal origin move to the other pole
- the same applies for chromosomes of maternal origin
- direction that each chromosome travels is random
- both crossing over and random segregation work to shuffle the
chromosomes and genes they carry
Genetic recombination is the re-assortment of the chromosomes and the genes
they carry through crossing over and random segregation and this contributes
greatly to variation among organisms
- variation among individuals of a species is important because certain
combinations of genes can help an organism survive better than other
combinations of genes can
Errors in Meiosis
- changes in the structure of a chromosome in gametes can have severe
consequences
- these mutations can be passed from one generation to the next when the
gamete combines with another gamete to form a zygote
Nondisjunction is the failure of chromosomes to separate properly during
meiosis resulting in the addition or deletion of one or more chromosomes form a
gamete
Trisomy is having an extra chromosome usually produced when a gamete with
an extra chromosome is fertilized by a normal gamete
- ie Down Syndrome
- there is an extra chromosome
Triploidy is a condition where an organism has three (3n) sets of chromosomes
- happens when a diploid gamete unites with a normal haploid gamete
- zygote contain 3 sets of chromosomes
Polyploids describes an organism that has more than two sets of chromosomes
- rare in animals
- common in plants
- ie seedless grapes and watermelon are polyploids
- plant breeders can cross a diploid male plant with a tetraploid (4n
chromosomes ) female plant
- result is seedless variety of watermelon
- offspring are sterile and therefore don’t contain seeds.
5.3
Chromosome Theory of Inheritance states that genes are located on
chromosomes, and chromosomes provide the bases for the segregation and
independent assortment of genes
Sex-Linked Inheritance
- some traits that are passed from one generation to the next depends on
the gender of the parent carrying the trait
- because the genes for these traits are located on the sex chromosomes
Sex-linked inheritance is the transfer of genes on the X or Y chromosome from
one generation to the next
- a gene that is located on the X chromosome only is called X-linked
- a gene that is located on the Y chromosome only is called Y-linked
- most of the known sex-linked traits are X-linked
- very few Y-linked traits are known, may be because the Y chromosome is
much smaller than X chromosome
- X-linked traits in humans include colour blindness and hemophilia
Chromosomes and Gene Expression
- males and females produce the same amounts of protein coded by genes
located on the X chromosome
- females have 2 copies of the X chromosome in every cell
- males only have one copy of the X chromosome in every cell
- one of the X chromosomes in each female cell is inactivated
- therefore different X chromosomes are active in different cells
Barr body is the inactivated X chromosome in female cell
Polygenic Inheritance
Polygenic inheritance is a pattern of inheritance where a trait is controlled by
more than one gene
Continuous variation is a range of variation in one trait resulting from the
protein products produced by many genes
- continuous variation can be defined as the variation among individuals in a
population in which there is a gradient of phenotypes for one trait
Modifier genes are genes that work along with other genes to control the
expression of a trait