Biol 1057 - Mitosis & Meosis Flashcards
Outline where genetic information is stored
DNA
Briefly define genome
- An organisms complete set of genes in a cell which usually doesn’t change
- Human genome is 3,2 billion bases
- Smallest genomes are found in simple unicellular species
Outline how many chromosomes we have and how we cope with it
- 1 chromosome = 4cm
- 46 chromosomes – this is approximately 1.84m of
DNA in every cell of your body - DNA exists in a highly condensed state within the cell nucleus
1000-fold more tightly packed than “naked” DNA - Achieved with the help of proteins, the majority called histones
- DNA & Histones
are collectively called
“chromatin”
Define a chromosome
- A chromatin segment carrying the genetic information
- Chromosomes carry genes
- Humans have 23 chromosomes pairs
- Homologous chromosomes carry same genes
but are not identical as nobody has two identical parents.
Define diploid
- Organisms/cells have with two full sets of chromosomes, one from the mother and one from the father
(2N), called homologous chromosomes
Outline the growth phase
Chromosomes and DNA packaging
- DNA complexed with histones to form nucleosomes which consists of 8 histone proteins around which DNA wraps
- Nucleosomes form beads on DNA string and nucleosomes fold to produce 30nm fiber which then forms loops
which are compressed and folded to produce wide fiber
- Tight coiling of 250nm fiber produces chromatid of a chromosome
Briefly outline how a chromosome works
- Each chromosome contains two identical parts called “sister-chromatids”
- Chromatids stay connected initially at the centromere
- Every chromosome gives rise to two identical chromosomes.
Outline interphase stage as a stage of mitosis
- Longest stage in cell cycle
- cell carry on their usual functions (G1)
- DNA & organelles duplicate in preparation for mitosis (S)
- after DNA replication and before mitosis (G2)
Outline prophase
- Chromosomes condense & become visible
- Centrioles separate to move to opposite poles of the cell to form mitotic spindle fibres
- Nuclear envelope & nucleolus break down
Outline prometaphase
- Centrosomes on opposite poles of the cell
- Each chromatid is attached to a spindle fiber
Outline metaphase
- Sister chromatids line up along the equator of the cell
- Spindle fibres released from poles attach to chromatids by centromere
Outline anaphase
STAGE REQUIRES ATP provided by respiration
- Centromere divides separating sister chromatids and become daughter chromosomes
- Spindle fibres contract and chromosomes (no longer chromatids) pulled to opposite poles
Outline telophase
- Chromosomes at each pole decondense
- Cytoplasm splits(CYTOKYNESIS) and nuclear envelopes form around each group of chromosomes
- 2 daughter cells created which are genetically identical
Briefly describe meiosis
Meiosis produces male and female gametes
- Produces haploid gametes (N)
- In males, meiosis is a part of
spermatogenesis, which occurs in the
testes and produces sperm.
- In females, meiosis is a part of oogenesis,which occurs in the ovaries & produces
eggs/oocytes.
- Upon fertilization the zygote (2N) undergoes mitosis and develops to a fetus (unborn child)
- After birth mitosis continues throughout life
for growth and repair.
Outline 1 way of how genetic variation occurs
Prophase I - Synapsis
Homologous chromosomes
(containing two sister-chromatids
each) pair, line up side-by-side
gene-by-gene.
Homologous Recombination
- Non-sister chromatids exchange genetic material.
- Crossover is in average is slightly more than two per
homologous pair in humans.
Sister chromatids may no longer be identical and
instead may have different combinations of alleles.
Outline a 2nd way of how genetic variation occurs
Homologous pairs are oriented randomly
- - The number of possible arrangements is related to the chromosome number (x)
and for two homologous chromosomes is 2x
- In humans, with 23 pairs of homologs (M & F), each gamete contains one of
8,388,608 (2to the power of 23) possible chromosome combinations, all equally likely.
- 3 chromosome pairs, 23= 8 chromosome arrangement combinations, 4 shown here
Outline the steps of meiosis 1
- DNA replicates once but nucleus divides twice
Prophase l - Tetrads form, and crossing-over occurs as chromosomes condense;
the nuclear envelope fragments. - Metaphase I
Tetrads align at the spindle equator. Elther homologue can face either pole. - Anaphase I
Homologues separate, and dyads move to poles.
Telophase I - Daughter nuclei are haplold, having received one duplicated chromosome from each homologous pair.
Outline the steps of meiosis 2
Prophase l
Chromosomes condense, and the nuclear envelope fragments.
Metaphase ll
The dyads align at spindle equator
Anaphase ll
Sister chromatids separate, becoming daughter chromosomes that move to the poles, centromeres split
n =2
Telophase Il
Four haploid daughter cells are genetically different from each other and from the parent cell.
Compare mitosis to meiosis
Mitosis happens in somatic
cells that build our bodies
Meiosis happens in germ cells
* specialized cells of different
origin to the somatic cells
* destined to undergo meiosis
and transfer genetic
information to off-springs.
* present in the testis or
ovaries.
Outline haploid nuclei in males packed into sperm
- Spermatogenesis initiates in germ cells called spermatogonia.
- The entire process takes place in testis
- The process from spermatogonium to sperm takes about 74 days.
Outline haploid nuclei in femals packed into oocytes
- Formation of the ovum (egg cell) is produced at the end of oogenesis in the
ovaries of the females - Egg cells are massive in comparison to the sperm, as they need to support the
growth of the new organism (e.g. proteins, organelles) - Only 1 out of 4 haploid cells will become an oocyte, which gets all the
cytoplasmic content. The other 3 haploid cells will eventually become tiny
polar bodies. - meiosis in females starts in about 5 months fetus, the process is paused at
prophase I - only after puberty a selected number of cells can continue meiosis as part of
the menstruation cycle - the large periods between initiation and completion of meiosis may lead to
nondisjunction as females age.
Outline how meiosis can go wrong
- nondisjunction occurs when chromosomes fail to separate at either
anaphase I or anaphase II - spindle fibers do not form properly or attach correctly to a
chromosome - Result is sperm or egg cell more or less copies of a particular
chromosome (aneuploidy)
Outline polypoidy and aneuploidy
- Females have two X
chromosomes (XX) - males have one X and one Y
chromosome (XY) - Polyploidy: when a cell has one
or more complete sets of extra
chromosomes - Aneuploidy: when a cell has
one or just few extra or missing
chromosomes e.g. trisomy,
monosomy.
Outline an autosomal trisomy
² Persons usually have three copies of chromosome 21
² The chances of a woman having such child increase rapidly with
age, starting at about age 40.
² facial features, a unique pattern of hand creases, short stature, fissured
tongue, a round head.
² Intelligence varies greatly: some children have profound mental impairment,
whereas others learn well.
² Many affected children die before their first birthdays, often because of
congenital heart defects.
² Above-average risk for leukemia and Alzheimer disease.
