Inheritance

Fertilization

Fertilization

• The fusion of two haploid gametes to form a diploid zygote

  • Provides the means for inheritance

• Haploid cells are used in fertilization to maintain a normal number of chromosomes —> avoids polyploidy

Progenitor source

• The mother or parent cell

  • Divides to produce identical daughter cells

Nuclear division

• DNA is separated into separate nuclei through PMAT (phases of mitosis/meiosis)

  • These phases evolve into each other —> identification of phases is somewhat subjective

• Exists in two forms: mitosis and meiosis

Cytokinesis

• The division of cytoplasm to produce two individual daughter cells

• Animal cells will use myosin and actin filaments to form a concentric ring around the cell’s centre

  • Filaments constrict to form a cleavage furrow

  • The cells are formed with centripetal (outside) force

• Plant cells have carbohydrate-rich vesicles form a row at the cell’s centre

  • Vesicles fuse to form a new cell plate with further fuses with original parent cell wall

  • Separation occurs using centrifugal (inside) force

Mitosis

• Form of asexual cell division occurring in most cells (somatic cells)

• Maintains the same number of chromosomes in two daughter nuclei (diploid cells)

• Malfunctions of mitosis can lead to cancers

Meiosis

• Form of nuclear division that only occurs in gametes

• Creates four daughter cells with half the number of chromosomes (haploid cells)

  • Requires two sets of cell division

• Malfunctions in meiosis can lead to aneuploidy conditions (wrong number of chromosomes)

Binary fission

• Form of asexual cell reproduction used by prokaryotes/cells with no nucleus

Chromosome

• Thread-like DNA structure composed of a very condensed DNA molecule

• Species are characterized by having the same chromosome number across individuals

• Replicated chromosomes can form a pair connected at the centromere

  • Replicated chromosomes consist of two sister chromatids

• Allows for easier movement (since they are more tightly wound)

Chromatids

• The two identical chromosomes that form a replicated chromosome can be referred to as chromatids

  • Once they are separated, they are no longer chromatids

Centromere

• Set of protein complexes (cohesions) that hold the sister chromatids together

• Binds to kinetochores during cell division

Chromatin

• A relatively uncondensed form of DNA; exists as euchromatin or heterochromatin

  • Euchromatin is loosely packaged (expresses active genes)

  • Heterochromatin is tightly packaged (contains inactive genes)

• Chromatin is wrapped around eight histone proteins

• DNA exists in this form for most of the cell cycle

  • Chromatin becomes more tightly wound and becomes chromosomes for cell division

Kinetochores

• Protein complexes at the end of microtubules

  • Associate with each sister chromatid at the centromere

• Depolymerize microtubule filaments (removes tubulins) to pull sister chromatids apart

  • The microtubule will shorten and pull chromatids to the cell poles

Microtubule spindles

• Filaments attaching centrosomes to kinetochores

  • Centrosomes are made of two centrioles each

Interphase

• Cell cycle phase where chromosomal DNA is replicated to form sister chromatids

• Cells spend most of their life in this phase

Prophase

• DNA supercoils (condenses) from chromatin to form chromosomes

• Nuclear membrane breaks down and nucleus dissolves

• Paired chromosomes (sister chromatids) move to opposite poles and form microtubule filaments

Metaphase

• Kinetochores attach to centromere and movement of microtubules move chromosomes to align at the cell’s centre

Anaphase

• Centromeres are removed and the sister chromatids split apart

• Shortening/contraction of microtubule filaments cause sister chromatids to move to the poles

Telophase

• Chromosomes decondense into chromatin

• Nuclear membrane reforms around the two DNA sets and cell division finishes

  • Cytokinesis occurs at the same time as later stages of cell division

Mitotic index

• Ratio between the number of cells undergoing mitosis and total number of cells

  • Helps determine proliferation status of tissue and cancer diagnosis

Diploid cells

• Contain a maternal and paternal version of each chromosome

• All adult cells are diploid during interphase and before mitosis/meiosis

• Includes instances of diploid replicated chromosomes

  • Chromosomes exist as sister chromatids but there are still maternal and paternal versions of each chromosome

Haploid cells

• Contain only one of each chromosome

• Exists after anaphase 1 or anaphase 2 of meiosis

Reduction division

• The formation of haploid cells from diploid cells —> only occurs during meiosis

• Allows for offspring of fertilization to have the correct number of chromosomes

Interkinesis

• A second growth phase between the two meiotic divisions

• No DNA replication occurs during this phase

Meiosis I

• Phase where reduction division occurs —> homologous pairs are separated

• Chromosomes will line up in homologous pairs (side by side) instead of ‘single-file’ like in mitosis

• Crossing over and random assortment occur during this phase

Homologous pairs

• The maternal and paternal versions of each chromosome (collectively)

• Replicated homologous pairs (tetrads/bivalents) are joined at the centromere

  • Allows for crossing over to occur

Chiasma

• The points where homologous chromosomes are connected to form tetrads or bivalents

  • Allows bivalents to line up side-by-side during meiosis I

Meiosis II

• Results in cells having only maternal or paternal version of chromosome

  • Halves chromosome number (2n —> n)

• Allows for four cells to be produced in total

Crossing over

• The exchange or breaking of DNA segments between bivalents to form a new combination of paternal and maternal DNA

  • The points of exchange are called chiasmata

  • There may be thousands of chiasmata per bivalent

Recombinants

• Bivalents that have gone through crossing over

• Have unique gene sequences and therefore allow for greater diversity

Synapsis

• The process where bivalents become connected at the centromeres

• Allows for crossing-over and better mobility of chromosomes

Random Assortment

• Different combinations of maternal and paternal chromosomes can be inherited

  • Offspring will only receive one homologous chromosome from a bivalent

Oogenesis

• Egg/ova production during meiosis II

• Most organelles are retained in one daughter cell while the others become polar bodies

  • Ensures ova has enough resources since sperm do not provide any of this

  • Polar bodies will act as protection to ova

• An instance of uneven cell division

Budding

• Asexual reproduction method employed by organisms such as yeast

  • The cell does not split equally; daughter cell will be much smaller than parent cell

  • Further growth occurs after cell division

Non-disjunction

• Failed separation of chromosomes during meiosis

  • All cells will contain an incorrect number of chromosomes

• May occur during anaphase I (impacts all daughter cells) or anaphase II (impacts two daughter cells)

• Has a high correlation with maternal age (although non-disjunction may also occur in male gametes)

Aneuploidy

• The presence of abnormal chromosome numbers

  • Trisomy is an extra chromosome and monosomy is a missing chromosome

• Can be detected through karyotyping where chromosomes are organized and visualized for inspection

  • Cells can be harvested from fetus to be analyzed

Down syndrome

• Trisomy on chromosome 21

  • One parent’s gamete had non-disjunction, resulting in an extra chromosome

Polyploidy

• The doubling of chromosome numbers each generation

Locus

• The location of a gene on the chromosome (‘gene address’)

  • Based on the band the gene is located on and the chromosome’s arms

Autosome

• Chromosomes that can form matched pairs with identical chromosomes

• Only sex chromosomes are not autosomes (heterosomes)

Zygosity

• The allele combination for any gene or trait; the combination of dominant and recessive alleles

  • Homozygous: alleles are both recessive or both dominant

  • Heterozygous: one dominant and one recessive allele

  • Hemizygous: having an allele on only one gene (only true for sex-linked traits)