Biology 242: LAB: GAMETOGENESIS (Spermatogenesis – Male; Oogenesis – Female)
TALKING POINTS for Reproductive Cell Division
I. Central Dogma of Molecular Biology
Even before the genetic code was fully revealed, Francis Crick articulated what became known as the Central
Dogma of molecular biology: a summary of the flow of information in cells and between generation of cells.
Central Dogma may be represented as follows:
Cell
Replication
Transcription
Translation
Division
DNA
DNA
RNA
Protein
Trait
II. Genetics Terminology
(Genotype)
(Phenotype)
A. –ploid (or ploidy) refers to the number of sets of chromosome a cell (or organism) has
1. Haploid (1n) – ONE set of chromosomes
2. Diploid (2n) – TWO sets of chromosomes
3. Triploid (3n); Tetraploid (4n); etc. Polyploid (multiple sets of chromosomes)
B. Chromatin / Chromosomes / Genes / Alleles / Traits / Genotype / Phenotype / Genome
1. DNA is in its relaxed, “ribbon-like” form of CHROMATIN in the non-dividing cell (Interphase)
(Chromatin is the entire complex of DNA that makes up a eukaryotic genome)
2. DNA is in its condensed, “tightly-coiled” form of CHROMOSOMES in the dividing cell
(Chromosomes are specific segments of DNA and may be single copies or double copies (dyads);
if in the dyad form, the chromosome consists of two sister chromatids each of whose kinetochore
is joined as the centromere. Once sister chromatids go through disjunction (as they do in Anaphase
of Mitosis), each individual chromatid is now called a (non-dyad) CHROMOSOME.
3. Homologous Chromosomes: Chromosomes of the same type (number, length, position of centromere, etc)
that contain the same genes in the same locations. Also called homologs.
4. Gene: A section of DNA that encodes information for making a protein (or a functional RNA molecule);
sometimes used loosely for any section of DNA under study.
5. Allele: A particular version of a gene; different expressions of the same gene. (Example: The allele for
black hair pigment and the allele for red hair pigment are two versions of the one Hair Pigment Gene)
a. Relationships between alleles: Dominant / Recessive; Co-Dominant; Incomplete Dominance; Pleiotropy
6. Trait: Expression of the allelic forms of the genes an organism has; also referred to as a characteristic.
7. Genotype: Sum total of genes possessed by an organism, whether or not all genes are expressed.
8. Phenotype: Total of physical/chemical/behavioral/psychological expressions of all genes an organism possesses.
9. Genome: All of the genes necessary to specify an organism’s COMPLETE array of characteristics.
10. Genetic code: set of rules relating DNA’s nucleotide sequence to the corresponding codons of mRNA and to the
amino acids they specifiy.
C. Mitosis / Meiosis – these are terms for the NUCLEAR EVENTS ONLY in the division of somatic cells
and reproductive cells (aka: “sex cells” or gametes) respectively.
D. Cytokinesis – this is the term for the CYTOPLASMIC EVENTS ONLY in the divisions of both somatic + sex cells
BE SURE TO UNDERSTAND: for any cell division, Mitosis must be followed by cytokinesis or
Meiosis must be followed by cytokinesis
(In other words: Cell Division = Mitosis (or Meiosis); rather Cell Division = Mitosis followed by Cytokinesis
or Cell Division = Meiosis followed by Cytokinesis
E. Somatic Cell Division (Mitosis and Cytokinesis) may be represented: 2n
2 2n cells
Sex Cell Division (Meiosis and Cytokinesis) may be represented: 2n
4 1n cells (male sex cells)
or for female gametogenesis: 2n
1 1n cell + 1 – 3 polar bodies
F. In Meiosis, there are two successive Divisions:
1. Meiosis I: Reduction Division (2n nucleus; divides into 2 1n nuclei, but chromosomes are still dyads)
2. Meiosis II: Equatorial Division (2 1n nuclei divide into 4 1n nuclei; functional single chromosomes)
G. Other useful terms: Tetrad; Synapsis; Crossing-Over; Jumping Genes; Linkage Group; Non-Disjunction
of Sister Chromatids; Trisomy; Autosomes; Sex Chromosomes; Geno-/Phenotype;
1.
Synapsis: The condition in which two members of a homologous pair (of chromosomes) lie together, genefor-gene, locus-for-locus in physical contact with each other; this momentary alignment enables crossing over
(exchange of part of a chromatid from one chromosome with the equivalent part of a chromatid from the homologous chromosome). While homologous pairs are aligned, they are called tetrads because they consist of 4
chromatids. Once out of synapsis, the homologous chromosomes (both dyads) still associate closely together
until Anaphase I when one of the homologs migrates away from the other homolog to an opposite pole.