Reproduction

Introduction

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I. Asexual Reproduction

II.a. Fission

Fission is by far the most common form of reproduction.

I.a-1.) Binary Fission

I.b-1A.) Irregular Binary Fission

I.b-1B.) Longitudinal Binary Fission

I.b-1C.) Transverse Binary Fission

I.b-1D.) Oblique Binary Fission

II.b. Mitosis

I.b-1.) Intermitotic Phase (Interphase or I-Phase)

I.b-1A.) 1^st Gap, Gap-1 (or Growth-1) Phase (G₁-Phase)

The first of three parts of interphase. The cell grows in preparation to replicate its DNA during the synthesis phase.

I.b-1B.) Synthesis Phase (S-Phase): DNA Replication

The chromosomes duplicate their genes, and each chromosome partially splits into a pair of identical sister chromatids which remain partially joined at the centromere (these sister chromatids will be pulled apart and become "daughter chromosomes" during the anaphase portion of the mitotic phase).

I.b-2.) Mitotic Phase (M-Phase)

I.b-2A.) Prophase

During prophase, the nucleolus disappears, the chromatin (loose DNA, RNA, and protein) inside the nucleus coils into distinct chromosomes, centrosomes separate, and microtubules (spindles fibers) form.

I.b-2B.) Prometaphase

The nuclear envelope begins to dissolve, each chromosome forms a pair of kinetochores at its centromere, and microtubules attach to the kinetochores.

I.b-2C.) Metaphase

Chromosomes line up along the metaphasic plate: an imaginary line or "equator" along which the cell will eventually divide. The lining up of the chromosomes along the metaphasic plate is caused by the chromosomes being pulled on evenly from either side by the spindles to which the kinetochores of the chromosomes are attached by microtubules.

I.b-2D.) Anaphase

The chromosomes are pulled apart, sister chromatid from sister chromatid, toward the poles of cell by the spindles/microtubules to form daughter chromosomes. Each daughter chromosome descends from a single chromatid, which is why each chromosome replicated its DNA and changed from a linear, single-chromatid structure with only one copy of each gene, into the more familiar, x-shaped pair of sister chromatids during the Synthesis Phase.

I.b-2E.) Telophase

Two new nuclear envelopes form, distinct chromosomes unfurl into chromatin (the "natural" or most common condition for nucleic acids), and a new nucleolus appears within each new nuclear evenlope (for a total of two nucleoli).

I.c-1.) Cytokinesis

After the mitosis of the cell nucleus into a pair of nuclei (think of a double-yolked egg), the cytoplasm ("egg white" in the double-yolked egg analogy) begins to divide in a manner superficially reminiscent of the binary fission of singe-celled organisms. The cell splits into daughter cells, each containing one of the two nuclei briefly formed during the mitosis of the parent or "mother" cell.

II. Sexual Reproduction

II.a. Meiosis

Most eukaryotes are capable of meiotic reproduction, even though the majority of us are unicellular organisms who for the most part reproduce mitotically.

Meiosis is reductional. One diploid cell becomes four haploid cells.

II.a-1.) Intermeiotic Phase (Interphase or I-Phase)

I.a-1A.) 1^st Gap, Gap-1 (or Growth-1) Phase (G₁-Phase)

The first of three parts of interphase. The cell grows in preparation to replicate its DNA during the synthesis phase.

I.a-1B.) Synthesis Phase (S-Phase): DNA Replication

The chromosomes duplicate their genes, and each chromosome partially splits into a pair of identical sister chromatids which remain partially joined at the centromere (these sister chromatids will be pulled apart and become "daughter chromosomes" during the anaphase portion of the mitotic phase).

I.a-2.) Meiosis I

I.a-2B.) Prophase I

Sinapsis: Chromosomes (each still with a pair of sister chromatids) wrap around each other.

Crossing over

Law of Segregation

I.a-1C.) Metaphase I

Independent orientation. There are 2ⁿ possible ways the chromosomes may orient with regard to one another. So if there are two chromosomes, they can orient in 2²=4 different ways, and three chromosomes would give us 2³=8 possible combinations. Humans have n=23 chromosomes in each haploid cell and 2n=46 chromosomes in each diploid cell, which gives use 2²³=8,388,608 possible chromosomal combinations.

Also during metaphase I, microtubules attach to kinetochores as in the metaphase of mitosis.

I.a-1C.) Anaphase I

Microtubules pull homologous chromosomes away from one another (but do not split individual chromosomes in twain as in the anaphase of mitosis).

I.a-1C.) Telophase I

I.a-1.) Meiosis II

I.a-1A.) Segregation Pattern 1

I.a-1A.) Segregation Pattern 2

II.b. Conjugation

II.c. Autogamy

Practiced by Paramecium aurelia (

III. Comparison of Reproductive Strategies

IV. Rules of Heredity

The science of heredity was greatly improved by the Mendel's study of pease plants ("pea" plants in modern, butchered English) and by Morgan's study of fruit flies.

IV.a. The Law of Independent Assortment

Each chromatid is a single DNA molecule.

IV. Units of Heredity

The science of heredity was greatly improved by the Mendel's study of peas and by Morgan's study of fruit flies.

IV.a-1.) The Chromatid

Each chromatid is a single DNA molecule.

IV.a-1.) The Telomere

IV.a-1.) The Centromere

IV.a-1.) The Germline

Germline cells, also known as "gametes", are those cells which multicellular eukaryotes use to pass hereditary information from one generation to the next. An admittedly very limited analogy would be the reproduction of eusocial insect colonies: An insect colony can grow and replenish its numbers through their normal, routine breeding practices just as multicellular organisms can grow and even re-grow damaged tissues through mitosis, but to spawn an entirely new colony, the king and queen insects must leave their respective colonies to find mates, just as the gametes of sexually-reproducing multicellular eukaryotes must (usually) find or be found by gametes from other multicellular eukaryotes of the sufficient genetic similarity (the "same species") in order to begin a new multicellular eukaryote (cell colony). The gametes are the kings and queens, and the germline the "royal line", of the cell colonies that are our bodies.

IV.a-1.) The Gamete

In most multicellular eukaryotes, the founding of a new cell colony (multicellular organism) is accomplished through sexual reproduction, which involves specialized "germline" cells known as gametes.

IV.a-1.) The Haploid

IV.a-1.) The Diploid

IV.a-1.) The Gene

The gene is an often loosely-defined, theoretical unit of heredity. Comparable in some ways to the linguistic concept of a phoneme, a gene can have multiple alleles just as a phoneme can have multiple allophones, or sounds (phones) that are equated and considered the same by the speaker (like the "th" in "with" and "this", or the "p" in "plastic" and "spoon"). The "main allele" for a given gene is normally considered to be either the most dominant allele, or, in the case of model organisms, the so-called "wild type" (basically whatever allele appeared most dominant in the first few individuals harvested from "the wild"), much as the "main" allophone for a given phoneme will be that used in whatever phonetic environment the speakers of a language consider most "typical". Note that alleles associated with dominant phenotypes are not all necessarily widespread, as such alleles may be associated with maladaptive phenotypes (harmful mutations) and will therefore be quickly suppressed by natural selection. Genes reside on chromosomes, according to the Chromosome Theory of Inheritance, and are made up of sequences of nucleotides.

IV.a-1.) The Homolog (or Homologue)

Homologous chromosome. The maternal and paternal copies of a particular chromosome are known as homologous chromosomes.

IV.a-1.) The Allele

Alleles are distinct forms or versions of a particular gene. In fact, a "gene" can be thought of as a set of alleles, in much the same way that a phoneme in linguistics can be thought of as a set of allophones.

IV.a-1.) The Genotype

IV.a-1.) The Phenotype

A phenotype is the observable manifestation of a genotype. It can be thought of as an interaction between the genotype and the external environment.

IV.a-1.) The Chromosome

V. Modes of Transmission

IV.a-1.) Homozygosity

IV.a-1.) Heterozygosity

IV.a-1.) Autosomal Transmission

IV.a-1.) Sex-linked Transmission

IV.a-1A.) X-linked Transmission

IV.a-1B.) Y-linked Transmission

IV.a-1.) Dominant Phenotypes

IV.a-1.) Recessive Phenotypes

If an organism is of a phenotype that that differs from that of either parent, and the phenotype can be observed elsewhere in the family and/or population, then this is considered to be a recessive phenotype. While it is conceptually possible that this could be caused by a novel mutation which just so happens to mimick an existing phenotype, this is highly unlikely (although we know it does occasionally happen, due to the number of different alleles for red hair in humans, or for albinism in multicellular eukaryotes, among other examples).