Week 4 Flashcards
Mitosis, Meiosis, and Chromosome Numbers
define: mitosis
the process of nuclear division in cell that produces daughter cells that are genetically identical to each other
what are the phases of interphase
- G1
- S
- G2
what are the cell division phases
mitosis and cytokinesis
what are the phases of mitosis
- prophase
- metaphase
- anaphase
- telophase
define: chromatin
the complex of DNA and protein found in a cell’s nucleus
what occurs in gap 1, G1
- cells actively produce many of the products specific to their biological role in the body
- skin stem cells divide rapidly to provide a continuous supply of keratinocytes that form the outer surface
what is G0
when G1 is the only phase, e.g. human nerve cells
what occurs in the synthesis phase
- duplication of chromosomes to produce identical sister chromatids
- sister chromatids remain joined at the centromere
what occurs in gap 2, G2
cell is busy synthesizing proteins necessary for mitosis and impending cell division
what is the state of chromosomes during interphase
chromosomes are duplicated during S phase but remain as diffuse chromatin
what occurs during prophase
- chromosomes condense, sister chromatids become visible
- centrosomes migrate to opposite poles
- degradation of nuclear membrane
- microtubules invade the nuclear area from opposite centrosomes and attach to chromosomes at the kinetochore
define: centrosomes
pairs of centrioles
define: kinetochore
site at which chromosome is attached to microtubules of the mitotic spindle
what occurs during metaphase
- chromosomes align on the metaphase plate
- sister chromatids face opposite poles
what occurs during anaphase
sister chromatids separate at centromere
define: disjunction
separated sister chromatids move to opposite poles
define: nondisjunction
unequal division of sister chromatids
what occurs during telophase
- two nuclear membranes and nucleoli reform
- spindle fibers disappear
- chromosomes uncoil and become a tangle of chromatin
what occurs during cytokinesis
cytoplasm divides, splitting the parent cell into two daughter cells
define: meiosis
the process of gamete formation that produces daughter cells that are genetically distinct from each other
what happens to the chromosome number during meiosis
reduced from diploid (2n) to haploid (n)
define: somatic cells
cells of the body other than germ cells
what does n represent
the number of chromosomes in a gamete
define: germ cells
pockets of specialized cells incorporated into ovaries and testes; undergo meiosis to form gametes
which cells are haploid in humans
gametes
what occurs during meiosis I
separation of chromosome pairs 2n to n
what occurs during meiosis II
separation of sister chromatids
what are the 5 stages of prophase I
- leptonema
- zygonema
- pachynema
- diplonema
- diakinesis
what occurs during leptonema
- chromosomes duplicate, thicken and become visible
- centrosomes begin to move to each pole and produce spindle fibres
what occurs during zygonema
- chromosomes are maximally condensed
- chromosomes pair with homologues
- homologous chromosomes from tetrads and “zipper” together in an intimate association with proteins forming the synaptonemal complex
what occurs during pachynema
crossing-over
define: crossing-over
genetic exchange between nonsister chromatids of a homologous pair occurs
where do recombination nodules appear during crossing-over
along the synaptonemal complex
what is the purpose of recombination nodules
facilitating the DNA exchange at various points
does crossing-over always occur
yes, in meiosis
is the site of crossing-over consistent
no varies from meiosis to meiosis
what is the average number of crossing over per chromosome
3-4
what occurs during diplonema
- synaptonemal complex dissolves
- tetrads of 4 chromatids appears to pull apart slightly, but remains connected at chiasmata which hold nonsister chromatids together
- maternal and paternal exchange of information results in new allelic combinations
what occurs during diakinesis
- chromatids thicken and shorten
- nuclear membrane breaks down and the spindles attach to kinetochores
- further condensation of chromatids occurs
- non-sister chromatids that have undergone crossing-over remain loosely associated at chiasmata
what occurs during metaphase I
- tetrads line up along the metaphase plate
- each chromosome of a homologous pair attaches to fibres from opposite poles
what occurs during anaphase I
- chiasmata are removed
- homologous chromosomes move to the opposite poles
what occurs during telophase I (and state of chromosomes)
- haploid number of chromosomes
- sister chromatids attached at centromere
- nuclear membranes form around the chromosomes that have moved to the poles
what occurs during prophase II
- chromosomes condense
- centrioles move to the poles
- sister chromatids attach to spindle fibers from opposite poles
define: oogenesis
the process of formation of female gametes
how is a primary oocyte formed
diploid germ cell undergoes mitosis
what is produced after a primary oocyte undergoes meiosis I
one secondary oocyte and one polar body
what happens to polar bodies
they disintegrate and get reabsorbed
what is produced after a secondary oocyte undergoes meiosis II
an ovum (egg) and a polar body
does the first polar body complete meiosis II
in some species, but it only produces polar bodies
what stage do stored oocytes in the ovaries arrest at
prophase I
what is the furthest stage released mature haploid oocytes complete
metaphase II
when do oocytes complete meiosis II
at fertilization
what is formed when an ovum and sperm fuse
diploid zygote
how is the embryo produced
zygote begins mitosis
what may cause the segregational errors that women older than 40 years have
long interval at arrested meiosis
what suggested that chromosomes are the agent of inheritance
mitosis and meiosis ensure a constant number of chromosomes in the somatic cells of a species, no other molecules is so conserved in each cell
who proved that chromosomes carry genes that determine traits
Thomas Hunt Morgan
what pattern is observed in X-linked traits
criss-cross inheritance, females inherit trait from father, males inherit trait from mother
define: hemizygous
only have one copy of a gene instead of the usual two
what did Morgan work with
drosophila with different eye colours
what is recent proof that genes reside on chromosomes
DNA sequencing
what is recent proof that genes determine traits
molecular cloning of new genes onto a chromosome imparts the new trait
what is recent proof that chromosomes are the vehicle of inheritance
nuclear cloning (e.g. Dolly)
what was the first human gene to be assigned to a particular chromosome
red-green colour-blindness, lies on the X chromosome
define: aneuploids
the loss or gain of one or more chromosomes
define: aneuploids
individuals whose chromosome number is not an exact multiple of the haploid number for the species
define: monosomic
individuals lacking one chromosome from the diploid number (2n-1)
define: trisomic
individuals having one chromosome in addition to the diploid set (2n+1)
define: tetrasomic
individuals having four copies of a particular chromosome (2n+2)
what is a characteristic of autosomal aneuploidy
generally deleterious to the organism
what type of aneuploidy of autosomal chromosomes is generally lethal in humans
monosomy
what trisomies have been recorded to survive in utero
trisomy 13, 18, and 21
define: gene dosage
number of copies of a gene present in a cell
why is having an extra X chromosome in females not detrimental
X-inactivation occurs
how many genes does X-inactivation affect
all but one X chromosome (except from some genes near telomere and centromere of short arm)
when does X-inactivation usually occur in mammalian species
around 16 days of gestation
define: Barr body
highly condensed body of an inactivated X chromosome within the cell
which X chromosome is usually inactivated between daughter cells after mitosis
the same one
why are females genetic mosaics
they are a 50/50 expression mix of father’s X genes and mother’s X genes
why are calicos almost always females
fur colour is X-linked so males can only be one colour, females are a mosaic of the two
why does only one X chromosome cause abnormality in Turner syndrome
in the normal female, the inactivate X contains some genes that are expresses and needed for viability
what is the phenotype of Turner syndrome
- short stature
- swollen hands and feet
- webbing of skin in the neck
- drooping eyelids
- cardiovascular abnormalities
- hearing impairment
- poor breast development
- narrow hips
- sterility
why does an extra X chromosome cause abnormality in Klinefelter syndrome
the genes that inactivated X’s still express are expressed at twice the level present in normal males
what is the phenotype of Klinefelter syndrome
- no frontal baldness
- poor beard growth
- narrow shoulders
- wide hips
- some breast development
- long legs
- small testes
- higher pitched voice
- reduced fertility
how would a male calico cat occur
if a Klinefelter cat has the black allele and the orange allele
can you get normal gametes from aneuploidy
yes
how can aneuploid mosaics form
from mitotic nondisjunction or chromosome loss
are true XOs viable
no, Turner syndrome females are mosaics
define: gynandromorph
organism that contains both male and female characteristics
how do gynandromorphs form in drosophila
when an XX female loses one of her X chromosomes during the first mitotic division after fertilization
define: euploids
species containing only complete set of chromosomes
define: monoploids
organisms which have only one complete set of chromosomes (usually infertile)
define: polyploids
euploids that carry three or more complete sets of chromosomes
what is an example of monoploidy
insects like ants, bees, and wasps
fertile individuals are diploid, but sterile males are produced are parthenogenesis
define: parthenogenesis
development of a unfertilized female gamete into an embryo
can parthenogenesis lead to a diploid individual
yes, e.g. komodo dragon
what gender can arise from parthenogenesis in komodo dragons
only male ZZ, WW is lethal (females are ZW)
how many sets of chromosomes do salmon carry
4, they are tetraploids
why are triploids almost always sterile
sexual reproduction results in unbalanced gametes, can’t evenly split 3 chromosomes
why are tetraploids usually sterile
synapsis is not uniform so gametes will be aneuploid
why are salmon able to be tetraploids
they have a tagging system that ensures equal pairing at synapsis
what do larger chromosome numbers often lead to in plants
increased size
what are the chromosomes numbers in seedless varieties
they are polyploid, e.g. seeded watermelon are diploid and seedless watermelon are triploid
why are hybrid species usually sterile
there is not enough DNA sequence similarity in chromosomes for them to pair up at meiosis I
