meiosis
meiosis is the process of cell division during which one cycle of chromosome replication is followed by
gametes
union of the two gametes in fertilization restores the chromosome number to 46
distribution of chromosomes in meiosis is an exact process
meiosis is a two-step process
both events are characterized by the same 4 stages or phases as that seen in mitosis
meiosis I:
before cells enter meiosis, chromosomes are
each replicated chromosome has a single centromere
during prophase I, chromosomes condense and homologous
chromosomes physically associated with one another
there is evidence that there is a physical exchange of chromosomal
material between homologues
during metaphase I, synapsed members of each chromosome pair
during anaphase I, members of each chromosome pair separate and
move to opposite sides of the cell.
during telophase I, chromosomes unwind slightly and cytokinesis occurs
overall, two major events take place during meiosis I
1) reduction –
2) crossing over –
Meiosis II begins with haploid cells and also proceeds through the same 4 phases
prophase II –
metaphase II –
anaphase II –
telophase II –
In the end, the result is four haploid daughter cells which are not genetically identical
to the parental cell
normally meiosis ensures that
on rare occasions, a child gets both copies of a chromosome from one parent
This condition is known as
this condition is caused by nondisjunction
Ex: Prader-Willi syndrome –
ex: Angelman syndrome –
so: both a maternal and paternal copy of chromosome 15 must be present
Nondisjunction is the leading cause of
can occur in either the first or second division with different consequences
can produce monosomic or trisomic gametes
also know that advanced maternal age (over 35 yrs old) is a risk factor
it has been determined that about 94% of nondisjunction events are
but why this increase in nondisjunction as females get old? What mechanisms
are controlling this increase?
one reason might be how long eggs are suspended in meiosis I.
metabolic errors or other mutations may damage the developing egg
aneuploidy incidence is higher for the sex chromosomes
ex: Turner syndrome (XO), Klinefelter syndrome (XXY), XYY syndrome
and XXX syndrome.
chromosomal abnormality has been found when cells are grown in the
laboratory and certain chemicals are added
see fragile sites
17 heritable sites have been identified
molecular nature is really unknown
ex: fragile-X syndrome – fragile site near tip of q arm of X chromosome
changes can influence the phenotype (genetically controlled, observable properties)
discuss the role of proteins in living systems and how changes within them can influence
the importance of proteins to living systems is reflected in their name:
protein is derived from Greek word proteios
proteins are the most numerous and multifunctional class of molecules
they are essential to all cellular structures and biological processes
one of the most important classes of proteins is enzymes
those molecules that act as catalysts
enzymes accelerate the rate of a chemical reaction
the 3-D shape of an enzyme generates an active site
substrates
conversion of molecular substrates into products by a chemical reaction
enzymatic reactions do not occur at random or in isolation
the sum of all biochemical reactions in a cell is called metabolism
in a metabolic pathway, the product of one reaction serves as the substrate for
the next reaction
if fail to carry out one reaction in a pathway, then all the following reactions are
halted
enzymes are usually named for their substrates, with the suffix “ase” added
ex:
what about mutations that cause the loss of activity in a single enzyme?
what phenotypic effect does this have?
build-up of one or more precursors in a pathway can be detrimental
lack of an essential component of a cellular process
overall: mutations that affect the action of enzymes can produce a wide range of phenotypic effects
prevalent diseases that are caused by metabolic mutations
1) phenylketonuria (PKU) –
the amino aicds that can’t be synthesized in the body are called
in the case of PKU, the essential amino acid phenylalanine, can’t be converted
phenylalanine accumulates in blood and urine because of a deficiency in
if untreated, affected individuals become mentally retarded and have trouble
with movement of arms and legs
how does the failure to convert phenylalanine to tyrosine produce these problems?
high levels of phenylalanine accumulate around cells of the developing nervous
system after birth and this blocks the uptake of
PKU can be treated by controling the amount of phenylalaninie consumed
have to balance the amount of phenylalanine
treatment must begin within 1-2 months of birth
some states require PKU screening of newborns…..is this appropriate?