One of the biggest discoveries in modern day genetics ... - LCVI

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14 Δεκ 2012 (πριν από 4 χρόνια και 7 μήνες)

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Cellular Reproduction and
Chromosomes

Cell Cycle, Mitosis, Meiosis and
Gene Expression

Fertilization


When reproduction occurs there are two
ways this can occur:


Asexual Reproduction


This involves no fertilization, as offspring is
produced by a single parent and inherit all the genes
and traits from the single parent


Sexual Reproduction


This involves fertilization of a female egg from
sperm cells of a male producing offspring of many
traits.


More on this later…

Cell Cycle


Cells reproduce through a continuous sequence of
growth and division known as the
cell cycle


Interphase


cells are making new molecules and
DNA is copied in a process called DNA
replication.


Interphase broken into 3 steps:


G1


Gap 1 involves metabolic activities


S


Synthesis involves DNA replication


G2


Gap 2 is preparation for cell division


M


Mitosis and Cell Division

Cell Cycle


Timing of each phase can vary among different
types of cells.


Organ cell vs. skin cell


Parent cell divides to create two new daughter
cells (somatic cells have 46 chromosomes)


Purpose of Mitosis

1.
Growth

2.
Regeneration of Damaged Tissue

3.
Maintenance of the Body

Chromosomes


Hold the genetic information needed to
maintain the cell and make new copies of
the cell


Made up of two sister chromatids which are
held together with a centromere.


Chromatids are exactly identical to one another

Phases of Mitosis


Several events must happen to ensure that as a
cell divides, the genetic material inside is
properly shared between each new daughter cell.


Root tip cells are often looked at as they undergo
mitosis frequently


Phase order:


PMAT


1.
Prophase

2.
Metaphase

3.
Anaphase

4.
Telophase



p. 88, fig. 4

Prestep!!!


INTERPHASE!


During this phase the cell
grows, heals, and creates
proteins in order to start the
division process!


The cell duplicates it’s
generic material (called
chromatin) and creates two
identical sister chromatids,
joined by an object known
as a centromere.

Mitosis


Prophase


chromatin, which is DNA and
proteins, condenses and
becomes visible


Nuclear membrane and
nucleolus disappear


Centrioles made up of
microtubules migrate to
opposite poles of the cell


Spindle fibres start to form
between the two centrioles


Mitosis


Metaphase


Spindle fibres attach to
centromere


Chromosomes line up on the
cell’s equator (equatorial
plate)



Anaphase


Centromere splits and
chromatids are pulled to
opposite poles of the cell


Mitosis


Telophase


Chromatids reach the two opposite poles


At this time each chromatid is
considered a single non
-
replicated
chromosome


Chromosomes unwind and become less
visible



Cytokinesis


Actual cell division


Spindle fibres disappear, nucleolus
reappears, nuclear membrane and in
plant cells a new cell wall is formed

Cell Clock


Why is it that if cells can continuously divide, how
come we can’t stay eternally young?


Research has shown that there is a specific time
frame, or biological clock, that regulates the
amount of divisions a cell can make.


Heart cells and the magic # of 50


Cell division is usually controlled by
specialization


i.e. Skin cells vs nerve cells


Cell Clock


There are only two types of cells that are able to
divide continuously


Spermatocytes (sperm producing cells)


From puberty to old age, spermatocytes are produced


When it eventually becomes a sperm cell and specializes, there
are no more divisions that occur


Cancer cells


Cancer cells grow so quickly that cell specialization does not
have time to occur.


i.e. Leukemia and white blood cells

Errors in Mitosis


Mutations are permanent errors in the normal
DNA molecule and can severely affect the mitotic
process


Mutagens such as toxic compounds, radiation or viruses
can lead to mutations



Mutations are passed on and only found in the
daughter cells of the initially affected cell.


FHIT gene affected by cigarette smoke will undergo mitosis
more frequently then normal and this leads to a tumour


Errors in Mitosis


Certain genes act like switches and can
produce proteins that will turn certain
processes like mitosis, on or off.


A mutation could permanently affect one of
these genes and leave a gene switched on
permanently


Genes that are activated by a mutation are
called oncogenes and will often lead to tumours


Cancer


Associated with many diseases but is based
around the uncontrolled, unregulated growth of
cells.


After cells in your body specialize, they are only
to divide to replace damaged cells.


There is a balance between cell death and cell
replication within the body to keep it healthy!!!


Cancer disrupts this balance!!!


Cancer


Cells usually don’t divide on their own


Cancer cells do!


Cancer cells can divide in culture about once every 24
hours. Not in living organisms though (thankfully!!!)


Cancer cells do not adhere to well to other cancer cells, nor
do they stick well to normal cells.


Metastasis!!!


Cancer cells lack the ability to mature and specialize:
Therefore another threat is that cancer cells cannot carry
out some of the functions of normal cells.


Causes of Cancer


Genetics plays a relatively small role in
predicting cancer (breast cancer is one of
the few cases where there is documented
evidence)



Lifestyle!!!


Cloning


Cloning is the process of forming IDENTICAL offspring
from a single cell or tissue.


For the most part, cloning does not result in the variation of
traits that would occur with the combination of male and
female sex cells.


Therefore what kind of reproduction is this?


Asexual



Think about how cloning of cells is similar to cancer
cells


What kind of clones can you have?


Plant Clones!!!!!


One of the biggest discoveries
in modern day genetics
occurred in 1958 by Fredrick
Stewart!


He created a full carrot from
a single carrot cell.


This was the first instance of
cloning and is now
commonplace in orchids.


Genetic Engineering


This discovery has lead to the process of
Genetic Engineering!


What is genetic engineering?


The process of intentional production of new
genetic material by substituting or altering
existing material.


However it does has it’s problems


Carrots, ferns, tobacco petunias and lettuce
clone well


But grass and legume families don’t!!!!!


What has been found is that some clones turn
into roots, and others to leaves, which each uses
different parts of DNA


What has resulted in the theory that to clone
plant cells well, the process of specialization/
differentiation must be delayed.


Animal Cloning


During plant cloning experiments, Briggs and King were
investigating nuclear transplants


They extracted the nucleus from an unfertilized egg of a
frog using a pipette, making the cell
enucleated.


Next the nucleus of a fertilized cell from the blastula stage
and the nucleus was placed into the first cell.


That cell began to act as a fertilized cell and the cell began
to replicate.


A nucleus that can bring a cell from egg to adult is referred
to as
totipotent
.


Cloning from Adult Animals


Originally, transferring nuclei from an adult
cell (specialized) into an enucleated cell
would not stimulate cell division


Why?


Until recently the only way to clone was to
split cells from embryos


However, some cells would still specialize
and full animals could not be cloned


Cloning


Production of identical copies of molecules,
genes, cells or even organisms.


Ian Wilmut in 1997 = Dolly


Egg from one adult female sheep, removed
nucleus.


Took nucleus from the mammary gland cell of
another sheep and inserted it into the original.


Egg was then implanted into the uterus of a
surrogate mother sheep

Cloning


Humans?


Any issues?


Insulin = Gene cloning


Genes that produce insulin are introduced into the DNA of
bacterial cells. Insulin is then manufactured by the bacteria which
we can harvest and use for diabetes patients.


PCR


Polymerase Chain Reaction = Gene Cloning


A single gene, or less, can be copied


Used to amplify or create many copies of DNA


Useful at crime scenes


Also useful to analyze ancient mummies DNA or in comparison of DNA of
extinct animals to those living today.

Meiosis, Chromosomes and
Heredity

Meiosis and Chromosomes


A zygote contains chromosomes from both parents
but it does not contain double the number of
chromosomes found in normal body cells. WHY?


Meiosis only occurs in reproductive organs and
produces cells known as gametes (eggs or sperm)
which are haploid (n).


All other cells (somatic) are diploid (2n) and contain
two copies of each type of chromosome.


Page 161, table 5.1

Chromosomes


The first part of meiosis reduces the chromosome
number from diploid to haploid, known as
reduction division.


Each sperm or egg cell contains 22 autosomes and
one sex chromosome (X or Y)


The autosomes control almost all of the functions of the
individual and the sex chromosomes determine the sex
of the individual.

Reduction Division

Meiosis


Almost the same as mitosis however there are two
sequences of each of the phases.



Interphase (see below)


Prophase I, Metaphase I, Anaphase I, and
Telophase I are all part of
reduction division
.


Prophase II, Metaphase II, Anaphase II and
Telophase II are identical to
mitosis
.



Interphase



chromosomes replicate during
interphase before cell division begins (sister
chromatids joined together by a centromere).

Meiosis


Prophase I



homologous chromosomes pair
which make up four chromatids called a tetrad.


Homologous chromosomes are similar but not identical
(like a pair of shoes) and each one has come from each
of your parents.


During pairing a process known as
crossing over

can
occur between non
-
sister chromatids.


This allows for recombination of genes and contributes
to genetic variation


Page 163, fig. 5.14


Crossing Over

Meiosis


Metaphase I



spindle fibres attach to the
centromere of each chromosome



Anaphase I

-

Homologous chromosomes are
separated independently


The centromere does not split



Telophase I



does not occur in all cells


If no, cell division goes directly to meiosis II


If yes, chromosomes uncoil and nuclear membrane is
formed (replication does not need to occur)


In females, meiosis II occurs after the egg is fertilized by a
sperm cell

Meiosis


Each cell beginning meiosis II is haploid


Each cell at the end of meiosis II is also haploid
although they are called gametes or spores.


The exact process of gamete formation is talked about
later



Meiosis II is exactly the same as Mitosis however
there are only 23 chromosomes to split at the
centromere in each cell instead of 46 as in the
somatic cells

Gamete Formation


Gametogenesis


process of creating sperm and
eggs.


Spermatogenesis


male gamete formation


Occurs in the testes


A diploid germ cell (spermatogonium) undergoes the
meiosis process to create 4 haploid cells


Following meiosis II, cytoplasm is lost and a tail
develops to allow locomotion


In some species this can occur year round while in other
species this is limited to certain times in the year.

Gamete Formation


Oogenesis


female gamete formation


Occurs in the ovaries


Diploid germ cell (oogonium) undergoes meiosis to create 1
haploid cell


After meiosis I, cytoplasm does not split equally and the
majority goes to the primary oocyte. The other cell is called a
polar body and it is not a viable sex cell.


After meiosis II, the cytoplasm is again unequally divided and
only one cell is viable as a sex cell (egg or ovum)


In humans meiosis I begins in the ovarian tissue of the embryo
before birth and does not continue beyond prophase I until
puberty.


Normally only one oogonium undergoes maturation each
month


Production of ova (many egg cell) continues from puberty until
menopause (between 40 and 50).


Genetic Variation


Dependant on 2 factors:

1.
Crossing over


occurs during prophase I and the
number of which is determined by the chromosome
size (usually 2 or 3 cross overs per chromosome).

2.
Random Segregation


how each pair of homologous
chromosomes line up during metaphase I is also
extremely important as that determines which pole
the chromosomes will go to.


These two factors work together and are the basis behind
genetic recombination


Page 171, fig 5.20

Errors in Meiosis


Nondisjunction



failure of chromosomes to separate properly


Results in the addition or deletion of one or more chromosomes
from a gamete


If a gamete with an extra chromosome is fertilized by a normal
gamete all the cells that develop from the zygote will also have
an extra chromosome =
trisomy


An example of this is Down’s Syndrome and this occurs when an
individual has an extra chromosome #21


If a diploid, rather then haploid, gamete unites with a normal
gamete the resulting zygote will have three sets of chromosomes
(3n) and this is referred to as
triploidy
.


Organisms that have more then 2 sets of chromosomes are called
polyploids

(rare in humans but common in plants such as
seedless varieties of fruit)

Human Genetic Analysis


Karyotype


illustration or photograph
of the chromosomes in the nucleus of a
somatic cell in an organism


46 chromosomes paired according to size,
shape and appearance.


Pedigree

= used to determine if an allele
is dominant, recessive, autosomal or sex
-
linked.