Biology
In a solution there are two parts
Solvent – that in which one dissolves something
Solute – the material dissolved in the above
Concentration – amount of material per unit volume (thus in a solution there are two, the
solvent and the solute.
When comparing solution concentrations – the concentration of solute outside the
membrane is compared with inside
Isotonic equal
Hypotonic lower
Hypertonic higher
Solvent is opposite
When there is a concentration gradient things move from high to low concentrations the
process is called diffusion (because of the random movement of molecules-Brownian
Motion). If water is moving it is called Osmosis.
Mitosis and Meiosis in Eukaryotic Cells
Mitosis - for nuclear division during cell replication (2N2N)
N= no, of chromosomes
Meiosis – process for nuclear division for gamete formation for sexual reproduction
(causes a halving of chromosomes in gametes (2N N)
Division of cytoplasm associated with above processes – cytokinesis)
Process of a single cell progressing to 2 cells – cell cycle
Cell cycle has 4 major phases
G1, S- phase, G2 & Mitosis (divided into 4 phases –Prophase, Metaphase, Anaphase
& Telophase)
S phase – DNA replication
G1- period before S phase
period)
G2- period after S phase
Known as Interphase (non dividing cells in this
After G2 cell enters a phase of Mitosis called Prophase
Prophase:
Nuclear membrane breaks down and replicated (2 chromatids)
chromosomes become visible (chromatids attached at the centromeres)
Prophase is followed by Metaphase
Metaphase:
Centrioles (MTOC’s) at the pole of the cell synthesize tubulin fibers that extend to the
cell equator
( known as the spindle fibers). Process aligns the duplicated chromosomes at the equator
because the spindle fibers are attached to the chromosomes at the centromere.
Chromosomes are arranged on the Metaphase Plate.
Anaphase:
Centromeres split separating the chromatids (now termed daughter chromosomes)
These daughter chromosomes are then pulled apart toward the poles of the cells by the
splindle fibers thus separating the daughter chromosomes.
Finally Telophase- the chromosomes uncondense, nucleoli reform along with nuclear
membranes around the separated chromosomes  2 nuclei.
Followed by cytokinesis separation of the cell into two.
Animals – formation of cleavage furrow – microfilaments across equator of cell contract
to pull membrane on opposite sides together generating two individual cells.
Plants – growth of cell wall across middle of cell.
Sexual Reproduction requires the fusion of two gametes (one from each parent ) to form
a zygote which can then undergo cell reproduction (Mitosis to form the organism). Each
gamete has one half the chromosomes required to make the zygote, so on fusion two
halves make a whole complement of chromosomes. The process of gamete formation
(Gametogenesis) is a specialized form of cell division and thus nuclear division
(Meiosis) resulting in halving of the chromosome number in the specialized cells called
gametes. Gametogenesis in males – Spermatogenesis & in females – Oogenesis.
Meiosis – two successive divisions of a single cell resulting in four cells ( 1, 2N cell  4,
1N cells)
There is no DNA replication between the divisions. A single DNA replication prior to the
first Division.
Two divisions are Meiosis I and Meiosis II.
Meiosis I : Four phases – Prophase, Metaphase, Anaphase & Telophase
Remember 2 of each chromosome – one from each parent. After DNA replication in S
phase each chromosome copied – chromatids ( attached at centromere).
Prophase I - each replicated chromosome pairs with its homolog (same chromosome
from other parent) and attached using a chiasmata (actually 4 chromatids)- called
SynapsisSpindle apparatus begins to form from centrosomes.
In Metaphase I - the homologous pairs move to the metaphase plate and align
themselves independantly (i.e. homologs can orient toward any pole).
Then in Anaphase I the Homologs (2 chromatids) migrate toward the poles of the cell.
Followed by Telophase I – reformation of nuclear envelopes and cytokinesis
Followed by a period of like interphase but no DNA replication – Interkinesis
Chromosomes are Haploid ( one chromosome from each homologous pair) but
replicated.
Prophase II: Nuclear membrane breaks down and spindle apparatus forms
Metaphase II – Haploid duplicated chromosomes align at the metaphase plate attaching
to fibers with centromeres.
Anaphase II – duplicated (or daughter Chromosomes) are pulled apart and move toward
the poles of the cell.
Telophase II- formation of nuclear membrane followed by cytokinesis
How does Meiosis apply to genetics in particular Mendelian Genetics.
In all sexually reproducing organisms the chromosomes occur in pairs called
homologous pairs one from each parent and thus theoretically have identical genetic
information. The cells are called somatic cells and are said to be Diploid.
Sex cells or gametes (produced in Meiosis) have only one of each chromosome and
are therefore said to be Haploid. Thus during sexual reproduction two gametes
combine to form a single cell zygote that obviously now have two copies of each
chromosome, one from each parent –Diploid.
Zygote can undergo mitosis for cell duplication.
Genetics is the study of how genetic information is passes from parents to offspring.
The foundation (particularly for sexual reproduction) of our understanding is attributed
to Gregor Mendel – hence Mendelian Genetics
The parts of genetic information that give specific characteristics are called genes
which are found at specific locations on the chromosomes called loci.
In Diploid cells ( the cells of sexually reproducing organisms) each gene is present in
two copies One from each parent on the specific homologous chromosome. While each
homologous chromosome has the same genes there are two forms or alleles.
If the alleles are identical the individual is said to be Homozygous. If the alleles are
different – heterozygous
Mendel took pure breeding strains of peas with different characteristics, mated them and
quantified the characteristics of the offspring using probability. Using large numbers of
crosses, producing many offspring he was able to predict what the expected offspring of
any given cross would be.
Probability- measurement of expected outcome Coin tossing example.
Monohybrid cross Homozygous green and Homozygous yellow - all yellow in the
next generation
How ever when these generations were crossed Green peas reappeared in specific ratios
3 yellow 1 green
These results allowed him to formulate the Law of segregation.
For every trait in an individual there are two factors (alleles) that separate and
recombine randomly during inheritance.
Meiosis is the process of separating the alleles and the random recombination is the
fusion of two gametes during fertilization. Each offspring produced has the random
chance of inheriting one of the two allele from each parent
Alleles
Dominant vs Recessive Remember this
Genotype (eg. All the genetic information from both parents)
vs. Phenotype (That information expressed- the trait you observed)
Codomindance – both alleles are expressed
Incomplete Dominance-blending of alleles (red & white become pink)
Continuous variation- a gradation in traits because more than one gene involved (
therefore for this trait the expression is polygenic (more than one gene)
What is the law of Segregation?
What is the law of independent Assortment?