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 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)
