The Role of Reproduction (Part 2)

8/2/2019 The Role of Reproduction (Part 2)

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The Role of Reproduction (Part 2)

Genetic Variation in Prokaryotes – Conjugation



Can

Differentiated Multicellular Organisms ReproduceAsexually?

• Yes!

o Asexual spore formation

o Seed formation

o Budding



o Parthenogenesis

Asexual Spores

Fungi• Asexual production

o Haploid spores

Sporangia

o Naked spores

Conidia

Apomixis• Asexual production of seeds

o Dandelions, citrus trees

o Produces seeds within the ovary without the mingling and

segregation of chromosomes

o Therefore the seeds are genetically identical to the parent

o Fertilization is NOT required

• Budding: outgrowth of a new cell from the surface of a parent cell;

new cells differentiate before the buds break away from the parents

Parthenogenesis:

The Development of Unfertilized Eggs



Grafting

Cloning Of Mammalian Embryos• Up to the blastocyst stage of development, all cells in a vertebrate

embryo are totipotential – each can potentially develop into an entire

new organism

• Therefore, separation of the cells leads to development of genetically

identical individuals



• The totipotent nature of

embryonic stem cells means that each cell can potentially developinto an entire new organism. Therefore, one way to achieve “cloning”



is to simply separate the cells and implant the resultant embryos atseparate sites (which happens naturally to give identical twins)

• Do not confuse embryonic stem cells with adult stem cells (e.g. bone

marrow and umbilical cord stem cells which are pluripotent, nottotipotent)

• This also means that you can pull out one cell from the early embryoand do genetic analysis on it, without harming development of therest of the embryo (pre-implantation genetic diagnosis)

• What about cloning of somatic cells?

1. Genetically engineered cloning of somatic cells

Involves activating certain genes in somatic cells that are

normally repressed, allowing those cells to revert to a primitive

state, from which they can differentiate totipotentially

Intranuclear transfer (Dolly the sheep)

Methodology: Intranuclear Transfer



Cloning’s Big Problem: Telomeres are Cellular BiologicalClocks• In somatic cells, lack of telomerase means that telomeres shorten

every cycle, ultimately killing the cell

o Preventing this (e.g. by overexpressing telomerase) may be

dangerous – cancer cells frequently continue to divide becausethey express telomerase

o Only the germ cells (e.g. the spermatogonia in the testes that

make sperm) normally have high telomerase, allowing them todivide indefinitely

Summary• Asexual reproduction: many different strategies (mitosis and fission

of single cells; replication by viruses within cells, asexual spore or

seed formation, budding, parthenogenesis)

• Asexual reproduction does not necessarily mean genetic invariability:

spontaneous mutations may confer advantage and be reproduced’

transduction and transformation allow for gene transfer

• Cloning results from either separation of embryonic stem cells,

resulting in more than one embryo with identical genetic properties

(e.g. identical twins), or transplant of a diploid somatic nucleus into amature ovum, in which the nucleus has been destroyed.

• Cloning by somatic nuclear transfer shows that somatic cell

differentiation does not result from loss of genes. The poor success,

however, shows that it is hard to reverse somatic cell differentiation.



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The Role of Reproduction (Part 2)