3.3 MEIOSIS Flashcards
G1 PHASE
increase cytoplasm volume, organelle production and protein synthesis (normal growth)
S PHASE
DNA replication
G2 PHASE
Increase cytoplasm volume, double the amount of organelle and protein synthesis (prepare for cell division)
PROPHASE I (6)
DNA supercoils and condenses. Chromosomes are visible under light microscope.
Nuclear membrane begins to break down and disintegrate.
The homologous chromosomes associate with each other to form
bivalenT.
Crossing over occurs: non-sister chromatids exchange genetic
information.
The crossing over point is called chiasma
Spindle fiber begins to form
METAPHASE I
Bivalents line up at the equator
Random Orientation occurs
Spindle fibers (microtubules) from each of the centrosomes attach to the centromere of bivalents.
RANDOM ORIENTATION
METAPHASE I
bivalents (homologous pairs) that come from the
mother or the father line up randomly on either side of the cell equator, independently of the other homologous pairs. Hence the daughter nuclei get a different mix of chromosomes.
ANAPHASE I
Contraction of the spindle fibers pulls homologous chromosome pair apart.
Chiasmata breaks apart and separate.
One chromosome of each pair move to opposite poles of the cell.
TELOPHASE I
Chromosome begins to uncoil and nuclear envelop reforms.
Chromosome number reduces from 2n (diploid) to n (haploid); however each
chromatid still has the replicated sister chromatid still attached (not homologous
pairs anymore).
Cytokinesis occurs and the cell splits into two separate cells.
PROPHASE II
Chromosomes condense again and become visible.
Spindle fibers again form.
Nuclear membrane disintegrates again.
METAPHASE II
Chromosomes line up along the equator.
Spindle fibre attaches to the centromere of the chromosome.
ANAPHASE II
Spindle fibers pull apart the centromeres and sister chromatids are pulled towards the opposite poles.
TELOPHASE II
Chromosomes arrive at opposite poles.
Nuclear envelope begins to develop around each of the four haploid cells.
Chromosomes begin to unwind to form chromatin.
Cytokinesis occurs and cells are split apart
GENETIC VARIATION IS DUE TO
crossing over in prophase I
CROSSING OVER
Prophase I of meiosis.
Crossing over creates new combinations of linked genes (genes on the same chromosome) from the mother and the father.
CROSSING OVER OCCURS BETWEEN…
non-sister chromatids of a particular chromosome.
CHIASMATA
Chiasmata are points where two homologous non-sister chromatids exchange genetic material during crossing over in meiosis
CROSSING OVER PRODUCT
When the chromatids are separated into different gametes after anaphase II, the gametes produced will not contain the same combination of alleles as
the parental chromosomes
CROSSING OVER CONSEQUENCE
This creates variation in the offspring regardless of random orientation.
WHEN DOES RANDOM ORIENTATION OCCUR?
Occurs in metaphase I of meiosis.
RANDOM ORIENTATION CONSEQUENCE
the genetic variation possibilities in the offspring is immeasurable.
NUMBER OF COMBINATIONS DUE TO RANDOM ORIENTATION
the number of combinations that can occur in the gamete is 2 to the power of n (n=number of chromosome pairs).
NUMBER OF COMBINATIONS DUE TO RANDOM ORIENTATION
the number of combinations that can occur in the gamete is 2 to the power of n (n=number of chromosome pairs).
NON- DISJUNCTION
an error in meiosis, where the chromosome pairs fail to split during cell division.
WHEN CAN NON- DISJUNCTION OCCUR
Non-disjunction can occur in anaphase I where the homologous pairs fail to split, or it can occur in anaphase II, where the sister chromatids fail to split.
NON- DISJUNCTION CONSEQUENCE
Too many chromosomes in a gamete cell or too few chromosomes in the final gamete cell.
NON- DISJUNCTION EXAMPLE
Down’s syndrome
DOWN SYNDROME
occurs when chromosome 21 fails to separate,
and one of the gametes ends up with an extra chromosome 21.
Therefore, a child that receives that gamete with an extra chromosome 21 will have 47 chromosomes in every cell.
DOWN SYNDROME AKA
Trisomy 21.
DOWN SYNDROME SYMPTOMS
Impairment in cognitive ability and physical growth, hearing loss, oversized tongue, shorter limbs and social
difficulties.
KARYOGRAM
A diagram or photograph of the chromosomes present in a nucleus arranged in homologous pairs of descending length.
USE OF KARYOGRAM
to make diagnosis of non-disjunction genetic disorder, such as Down’s Syndrome
AMNIOCENTESIS
a sample of the amniotic fluid surrounding the baby is removed using a syringe.
KARYOGRAM AND AMNIOCENTESIS
The sample contains skin cell from the baby, so we can use that to make a karyogram, in order to check for genetic disorder.
