Information

7.11D: Prokaryotic Reproduction - Biology

7.11D: Prokaryotic Reproduction - Biology


We are searching data for your request:

Forums and discussions:
Manuals and reference books:
Data from registers:
Wait the end of the search in all databases.
Upon completion, a link will appear to access the found materials.

Prokaryotes reproduce asexually by binary fission; they can also exchange genetic material by transformation, transduction, and conjugation.

LEARNING OBJECTIVES

Distinguish among the types of reproduction in prokaryotes

Key Points

  • Binary fission is a type of reproduction in which the chromosome is replicated and the resultant prokaryote is an exact copy of the parental prokaryate, thus leaving no opportunity for genetic diversity.
  • Transformation is a type of prokaryotic reproduction in which a prokaryote can take up DNA found within the environment that has originated from other prokaryotes.
  • Transduction is a type of prokaryotic reproduction in which a prokaryote is infected by a virus which injects short pieces of chromosomal DNA from one bacterium to another.
  • Conjugation is a type of prokaryotic reproduction in which DNA is transferred between prokaryotes by means of a pilus.

Key Terms

  • transformation: the alteration of a bacterial cell caused by the transfer of DNA from another, especially if pathogenic
  • transduction: horizontal gene transfer mechanism in prokaryotes where genes are transferred using a virus
  • binary fission: the process whereby a cell divides asexually to produce two daughter cells
  • conjugation: the temporary fusion of organisms, especially as part of sexual reproduction
  • pilus: a hairlike appendage found on the cell surface of many bacteria

Reproduction

Reproduction in prokaryotes is asexual and usually takes place by binary fission. The DNA of a prokaryote exists as as a single, circular chromosome. Prokaryotes do not undergo mitosis; rather the chromosome is replicated and the two resulting copies separate from one another, due to the growth of the cell. The prokaryote, now enlarged, is pinched inward at its equator and the two resulting cells, which are clones, separate. Binary fission does not provide an opportunity for genetic recombination or genetic diversity, but prokaryotes can share genes by three other mechanisms.

In transformation, the prokaryote takes in DNA found in its environment that is shed by other prokaryotes. If a nonpathogenic bacterium takes up DNA for a toxin gene from a pathogen and incorporates the new DNA into its own chromosome, it, too, may become pathogenic. In transduction, bacteriophages, the viruses that infect bacteria, sometimes also move short pieces of chromosomal DNA from one bacterium to another. Transduction results in a recombinant organism. Archaea are not affected by bacteriophages, but instead have their own viruses that translocate genetic material from one individual to another. In conjugation, DNA is transferred from one prokaryote to another by means of a pilus, which brings the organisms into contact with one another. The DNA transferred can be in the form of a plasmid or as a hybrid, containing both plasmid and chromosomal DNA.

Reproduction can be very rapid: a few minutes for some species. This short generation time, coupled with mechanisms of genetic recombination and high rates of mutation, result in the rapid evolution of prokaryotes, allowing them to respond to environmental changes (such as the introduction of an antibiotic) very rapidly.


RNA Polymerase is the enzyme that produces the mRNA molecule (just like DNA polymerase produced a new DNA molecule during DNA replication). Prokaryotes use the same RNA polymerase to transcribe all of their genes. In E. coli, the polymerase is composed of five polypeptide subunits. These subunits assemble every time a gene is transcribed, and they disassemble once transcription is complete. Each subunit has a unique role (which you do not need to memorize). The polymerase comprised of all five subunits is called the holoenzyme.

Transcription in prokaryotes (and in eukaryotes) requires the DNA double helix to partially unwind in the region of mRNA synthesis. The region of unwinding is called a transcription bubble. The DNA sequence onto which the proteins and enzymes involved in transcription bind to initiate the process is called a promoter. In most cases, promoters exist upstream of the genes they regulate. The specific sequence of a promoter is very important because it determines whether the corresponding gene is transcribed all of the time, some of the time, or hardly at all. The structure and function of a prokaryotic promoter is relatively simple (Figure 1). One important sequence in the prokaryotic promoter is located 10 bases before the transcription start site (-10) and is commonly called the TATA box.

Figure 1 The general structure of a prokaryotic promoter.

To begin transcription, the RNA polymerase holoenzyme assembles at the promoter. The dissociation of σ allows the core enzyme to proceed along the DNA template, synthesizing mRNA by adding RNA nucleotides according to the base pairing rules, similar to the way a new DNA molecule is produced during DNA replication. Only one of the two DNA strands is transcribed. The transcribed strand of DNA is called the template strand because it is the template for mRNA production. The mRNA product is complementary to the template strand and is almost identical to the other DNA strand, called the non-template strand, with the exception that RNA contains a uracil (U) in place of the thymine (T) found in DNA. Like DNA polymerase, RNA polymerase adds new nucleotides onto the 3′-OH group of the previous nucleotide. This means that the growing mRNA strand is being synthesized in the 5′ to 3′ direction. Because DNA is anti-parallel, this means that the RNA polymerase is moving in the 3′ to 5′ direction down the template strand (Figure 2).


Types of Sexual and Asexual Reproduction

Asexual and sexual reproduction, two methods of reproduction among animals, produce offspring that are clones or genetically unique.

Learning Objectives

Discuss sexual and asexual reproduction methods

Key Takeaways

Key Points

  • Asexual reproduction includes fission, budding, fragmentation, and parthenogenesis, while sexual reproduction is achieved through the combination of reproductive cells from two individuals.
  • The ability of a species to reproduce through fragmentation depends on the size of part that breaks off, while in binary fission, an individual splits off and forms two individuals of the same size.
  • Budding may lead to the production of a completely new adult that forms away from the original body or may remain attached to the original body.
  • Observed in invertebrates and some vertebrates, parthenogenesis produce offspring that may be either haploid or diploid.
  • Sexual reproduction, the production of an offspring with a new combination of genes, may also involve hermaphroditism in which an organism can self-fertilize or mate with another individual of the same species.

Key Terms

  • binary fission: the process whereby a cell divides asexually to produce two daughter cells
  • hermaphroditism: having sexual organs of both sexes
  • parthenogenesis: a form of asexual reproduction where growth and development of embryos occur without fertilization

Methods of Reproduction: Asexual & Sexual

Asexual Reproduction

Asexual reproduction produces offspring that are genetically identical to the parent because the offspring are all clones of the original parent. This type of reproduction occurs in prokaryotic microorganisms (bacteria) and in some eukaryotic single-celled and multi-celled organisms. Animals may reproduce asexually through fission, budding, fragmentation, or parthenogenesis.

Fission

Fission, also called binary fission, occurs in prokaryotic microorganisms and in some invertebrate, multi-celled organisms. After a period of growth, an organism splits into two separate organisms. Some unicellular eukaryotic organisms undergo binary fission by mitosis. In other organisms, part of the individual separates, forming a second individual. This process occurs, for example, in many asteroid echinoderms through splitting of the central disk. Some sea anemones and some coral polyps also reproduce through fission.

Fission: Coral polyps reproduce asexually by fission, where an organism splits into two separate organisms.

Budding

Budding is a form of asexual reproduction that results from the outgrowth of a part of a cell or body region leading to a separation from the original organism into two individuals. Budding occurs commonly in some invertebrate animals such as corals and hydras. In hydras, a bud forms that develops into an adult, which breaks away from the main body whereas in coral budding, the bud does not detach and multiplies as part of a new colony.

Budding: Hydra reproduce asexually through budding, where a bud forms that develops into an adult and breaks away from the main body.

Fragmentation

Fragmentation is the breaking of the body into two parts with subsequent regeneration. If the animal is capable of fragmentation, and the part is big enough, a separate individual will regrow.

Many sea stars reproduce asexually by fragmentation. For example, if the arm of an individual sea star is broken off it will regenerate a new sea star. Fishery workers have been known to try to kill the sea stars that eat their clam or oyster beds by cutting them in half and throwing them back into the ocean. Unfortunately for the workers, the two parts can each regenerate a new half, resulting in twice as many sea stars to prey upon the oysters and clams. Fragmentation also occurs in annelid worms, turbellarians, and poriferans.

Fragmentation: Sea stars can reproduce through fragmentation. The large arm, a fragment from another sea star, is developing into a new individual.

Note that in fragmentation, there is generally a noticeable difference in the size of the individuals, whereas in fission, two individuals of approximately the same size are formed.

Parthenogenesis

Parthenogenesis is a form of asexual reproduction where an egg develops into a complete individual without being fertilized. The resulting offspring can be either haploid or diploid, depending on the process and the species. Parthenogenesis occurs in invertebrates such as water fleas, rotifers, aphids, stick insects, some ants, wasps, and bees. Bees use parthenogenesis to produce haploid males (drones) and diploid females (workers). If an egg is fertilized, a queen is produced. The queen bee controls the reproduction of the hive bees to regulate the type of bee produced.

Some vertebrate animals, such as certain reptiles, amphibians, and fish, also reproduce through parthenogenesis. Although more common in plants, parthenogenesis has been observed in animal species that were segregated by sex in terrestrial or marine zoos. Two Komodo dragons, a bonnethead shark, and a blacktip shark have produced parthenogenic young when the females have been isolated from males.

Sexual Reproduction

Sexual reproduction is the combination of (usually haploid, or having a single set of unpaired chromosomes) reproductive cells from two individuals to form a third (usually diploid, or having a pair of each type of chromosome) unique offspring. Sexual reproduction produces offspring with novel combinations of genes. This can be an adaptive advantage in unstable or unpredictable environments. As humans, we are used to thinking of animals as having two separate sexes, male and female, determined at conception. However, in the animal kingdom, there are many variations on this theme.

Hermaphroditism

Hermaphroditism occurs in animals where one individual has both male and female reproductive parts. Invertebrates, such as earthworms, slugs, tapeworms and snails, are often hermaphroditic. Hermaphrodites may self-fertilize or may mate with another of their species, fertilizing each other and both producing offspring. Self fertilization is common in animals that have limited mobility or are not motile, such as barnacles and clams.


DMCA Complaint

If you believe that content available by means of the Website (as defined in our Terms of Service) infringes one or more of your copyrights, please notify us by providing a written notice (“Infringement Notice”) containing the information described below to the designated agent listed below. If Varsity Tutors takes action in response to an Infringement Notice, it will make a good faith attempt to contact the party that made such content available by means of the most recent email address, if any, provided by such party to Varsity Tutors.

Your Infringement Notice may be forwarded to the party that made the content available or to third parties such as ChillingEffects.org.

Please be advised that you will be liable for damages (including costs and attorneys’ fees) if you materially misrepresent that a product or activity is infringing your copyrights. Thus, if you are not sure content located on or linked-to by the Website infringes your copyright, you should consider first contacting an attorney.

Please follow these steps to file a notice:

You must include the following:

A physical or electronic signature of the copyright owner or a person authorized to act on their behalf An identification of the copyright claimed to have been infringed A description of the nature and exact location of the content that you claim to infringe your copyright, in sufficient detail to permit Varsity Tutors to find and positively identify that content for example we require a link to the specific question (not just the name of the question) that contains the content and a description of which specific portion of the question – an image, a link, the text, etc – your complaint refers to Your name, address, telephone number and email address and A statement by you: (a) that you believe in good faith that the use of the content that you claim to infringe your copyright is not authorized by law, or by the copyright owner or such owner’s agent (b) that all of the information contained in your Infringement Notice is accurate, and (c) under penalty of perjury, that you are either the copyright owner or a person authorized to act on their behalf.

Send your complaint to our designated agent at:

Charles Cohn Varsity Tutors LLC
101 S. Hanley Rd, Suite 300
St. Louis, MO 63105


DMCA Complaint

If you believe that content available by means of the Website (as defined in our Terms of Service) infringes one or more of your copyrights, please notify us by providing a written notice (“Infringement Notice”) containing the information described below to the designated agent listed below. If Varsity Tutors takes action in response to an Infringement Notice, it will make a good faith attempt to contact the party that made such content available by means of the most recent email address, if any, provided by such party to Varsity Tutors.

Your Infringement Notice may be forwarded to the party that made the content available or to third parties such as ChillingEffects.org.

Please be advised that you will be liable for damages (including costs and attorneys’ fees) if you materially misrepresent that a product or activity is infringing your copyrights. Thus, if you are not sure content located on or linked-to by the Website infringes your copyright, you should consider first contacting an attorney.

Please follow these steps to file a notice:

You must include the following:

A physical or electronic signature of the copyright owner or a person authorized to act on their behalf An identification of the copyright claimed to have been infringed A description of the nature and exact location of the content that you claim to infringe your copyright, in sufficient detail to permit Varsity Tutors to find and positively identify that content for example we require a link to the specific question (not just the name of the question) that contains the content and a description of which specific portion of the question – an image, a link, the text, etc – your complaint refers to Your name, address, telephone number and email address and A statement by you: (a) that you believe in good faith that the use of the content that you claim to infringe your copyright is not authorized by law, or by the copyright owner or such owner’s agent (b) that all of the information contained in your Infringement Notice is accurate, and (c) under penalty of perjury, that you are either the copyright owner or a person authorized to act on their behalf.

Send your complaint to our designated agent at:

Charles Cohn Varsity Tutors LLC
101 S. Hanley Rd, Suite 300
St. Louis, MO 63105


The Prokaryotic Cell

Figure 2. The features of a typical prokaryotic cell are shown.

Recall that prokaryotes (Figure 2) are unicellular organisms that lack organelles or other internal membrane-bound structures. Therefore, they do not have a nucleus but instead generally have a single chromosome—a piece of circular, double-stranded DNA located in an area of the cell called the nucleoid. Most prokaryotes have a cell wall outside the plasma membrane.

Recall that prokaryotes are divided into two different domains, Bacteria and Archaea, which together with Eukarya, comprise the three domains of life (Figure 3).

Figure 3. Bacteria and Archaea are both prokaryotes but differ enough to be placed in separate domains. An ancestor of modern Archaea is believed to have given rise to Eukarya, the third domain of life. Archaeal and bacterial phyla are shown the evolutionary relationship between these phyla is still open to debate.

The composition of the cell wall differs significantly between the domains Bacteria and Archaea. The composition of their cell walls also differs from the eukaryotic cell walls found in plants (cellulose) or fungi and insects (chitin). The cell wall functions as a protective layer, and it is responsible for the organism’s shape. Some bacteria have an outer capsule outside the cell wall. Other structures are present in some prokaryotic species, but not in others (Table 1). For example, the capsule found in some species enables the organism to attach to surfaces, protects it from dehydration and attack by phagocytic cells, and makes pathogens more resistant to our immune responses. Some species also have flagella (singular, flagellum) used for locomotion, and pili (singular, pilus) used for attachment to surfaces. Plasmids, which consist of extra-chromosomal DNA, are also present in many species of bacteria and archaea.

Characteristics of phyla of Bacteria are described in Figure 4 and Figure 5 Archaea are described in Figure 6.

Figure 4. Phylum Proteobacteria is one of up to 52 bacteria phyla. Proteobacteria is further subdivided into five classes, Alpha through Epsilon. (credit “Rickettsia rickettsia”: modification of work by CDC credit “Spirillum minus”: modification of work by Wolframm Adlassnig credit “Vibrio cholera”: modification of work by Janice Haney Carr, CDC credit “Desulfovibrio vulgaris”: modification of work by Graham Bradley credit “Campylobacter”: modification of work by De Wood, Pooley, USDA, ARS, EMU scale-bar data from Matt Russell)

Figure 5. Chlamydia, Spirochetes, Cyanobacteria, and Gram-positive bacteria are described in this table. Note that bacterial shape is not phylum-dependent bacteria within a phylum may be cocci, rod-shaped, or spiral. (credit “Chlamydia trachomatis”: modification of work by Dr. Lance Liotta Laboratory, NCI credit “Treponema pallidum”: modification of work by Dr. David Cox, CDC credit “Phormidium”: modification of work by USGS credit “Clostridium difficile”: modification of work by Lois S. Wiggs, CDC scale-bar data from Matt Russell)

Figure 6. Archaea are separated into four phyla: the Korarchaeota, Euryarchaeota, Crenarchaeota, and Nanoarchaeota. (credit “Halobacterium”: modification of work by NASA credit “Nanoarchaeotum equitans”: modification of work by Karl O. Stetter credit “korarchaeota”: modification of work by Office of Science of the U.S. Dept. of Energy scale-bar data from Matt Russell)

The Plasma Membrane

The plasma membrane is a thin lipid bilayer (6 to 8 nanometers) that completely surrounds the cell and separates the inside from the outside. Its selectively permeable nature keeps ions, proteins, and other molecules within the cell and prevents them from diffusing into the extracellular environment, while other molecules may move through the membrane. Recall that the general structure of a cell membrane is a phospholipid bilayer composed of two layers of lipid molecules. In archaeal cell membranes, isoprene (phytanyl) chains linked to glycerol replace the fatty acids linked to glycerol in bacterial membranes. Some archaeal membranes are lipid monolayers instead of bilayers (Figure 7).

Figure 7. Archaeal phospholipids differ from those found in Bacteria and Eukarya in two ways. First, they have branched phytanyl sidechains instead of linear ones. Second, an ether bond instead of an ester bond connects the lipid to the glycerol.

The Cell Wall

The cytoplasm of prokaryotic cells has a high concentration of dissolved solutes. Therefore, the osmotic pressure within the cell is relatively high. The cell wall is a protective layer that surrounds some cells and gives them shape and rigidity. It is located outside the cell membrane and prevents osmotic lysis (bursting due to increasing volume). The chemical composition of the cell walls varies between archaea and bacteria, and also varies between bacterial species.

Bacterial cell walls contain peptidoglycan, composed of polysaccharide chains that are cross-linked by unusual peptides containing both L- and D-amino acids including D-glutamic acid and D-alanine. Proteins normally have only L-amino acids as a consequence, many of our antibiotics work by mimicking D-amino acids and therefore have specific effects on bacterial cell wall development. There are more than 100 different forms of peptidoglycan. S-layer (surface layer) proteins are also present on the outside of cell walls of both archaea and bacteria.

Bacteria are divided into two major groups: Gram positive and Gram negative, based on their reaction to Gram staining. Note that all Gram-positive bacteria belong to one phylum bacteria in the other phyla (Proteobacteria, Chlamydias, Spirochetes, Cyanobacteria, and others) are Gram-negative. The Gram staining method is named after its inventor, Danish scientist Hans Christian Gram (1853–1938). The different bacterial responses to the staining procedure are ultimately due to cell wall structure. Gram-positive organisms typically lack the outer membrane found in Gram-negative organisms (Figure 8). Up to 90 percent of the cell wall in Gram-positive bacteria is composed of peptidoglycan, and most of the rest is composed of acidic substances called teichoic acids . Teichoic acids may be covalently linked to lipids in the plasma membrane to form lipoteichoic acids. Lipoteichoic acids anchor the cell wall to the cell membrane. Gram-negative bacteria have a relatively thin cell wall composed of a few layers of peptidoglycan (only 10 percent of the total cell wall), surrounded by an outer envelope containing lipopolysaccharides (LPS) and lipoproteins. This outer envelope is sometimes referred to as a second lipid bilayer. The chemistry of this outer envelope is very different, however, from that of the typical lipid bilayer that forms plasma membranes.

Art Connection

Figure 8. Gram-positive and -negative bacteria (credit: modification of work by “Franciscosp2″/Wikimedia Commons)

Bacteria are divided into two major groups: Gram positive and Gram negative. Both groups have a cell wall composed of peptidoglycan: in Gram-positive bacteria, the wall is thick, whereas in Gram-negative bacteria, the wall is thin. In Gram-negative bacteria, the cell wall is surrounded by an outer membrane that contains lipopolysaccharides and lipoproteins. Porins are proteins in this cell membrane that allow substances to pass through the outer membrane of Gram-negative bacteria. In Gram-positive bacteria, lipoteichoic acid anchors the cell wall to the cell membrane.

Which of the following statements is true?

  1. Gram-positive bacteria have a single cell wall anchored to the cell membrane by lipoteichoic acid.
  2. Porins allow entry of substances into both Gram-positive and Gram-negative bacteria.
  3. The cell wall of Gram-negative bacteria is thick, and the cell wall of Gram-positive bacteria is thin.
  4. Gram-negative bacteria have a cell wall made of peptidoglycan, whereas Gram-positive bacteria have a cell wall made of lipoteichoic acid.

Archaean cell walls do not have peptidoglycan. There are four different types of Archaean cell walls. One type is composed of pseudopeptidoglycan, which is similar to peptidoglycan in morphology but contains different sugars in the polysaccharide chain. The other three types of cell walls are composed of polysaccharides, glycoproteins, or pure protein.

Table 1. Structural Differences and Similarities between Bacteria and Archaea
Structural Characteristic Bacteria Archaea
Cell type Prokaryotic Prokaryotic
Cell morphology Variable Variable
Cell wall Contains peptidoglycan Does not contain peptidoglycan
Cell membrane type Lipid bilayer Lipid bilayer or lipid monolayer
Plasma membrane lipids Fatty acids Phytanyl groups


Multiple fission

Some algae, some protozoans, and the true slime molds ( Myxomycetes) regularly divide by multiple fission. In such cases the nucleus undergoes several mitotic divisions, producing a number of nuclei. After the nuclear divisions are complete, the cytoplasm separates, and each nucleus becomes encased in its own membrane to form an individual cell. In the Myxomycetes, the fusion of two haploid gametes or the fusion of two or more diploid zygotes (the structures that result from the union of two sex cells) results in the formation of a plasmodium—a motile, multinucleate mass of cytoplasm. The nuclei are in a syncytium, that is, there are no cell boundaries, and the nuclei flow freely in the motile plasmodium. As it feeds, the plasmodium enlarges, and the nuclei divide synchronously about once every 24 hours. The plasmodium may become very large, with millions of nuclei, but, ultimately, when conditions are right, it forms a series of small bumps, each of which becomes a small, fruiting body (a structure that bears the spores). During this process the nuclei undergo meiosis, and the final haploid nuclei are then isolated into uninucleate spores (reproductive bodies).


Transcription in Prokaryotes and Eukaryotes | Genetics

DNA dependent RNA polymerase is the single enzyme that catalyses the transcription of all types of bacterial RNA.

RNA polymerase binds to promoter and initiates transcription. It associates transiently with ‘initiation factor’ (σ) and uses nucleoside triphosphates as substrate, obeying the rule of complementarity and polymerises in a template depended fashion. It also facilitates the opening of the helix and continues elongation.

Chain elongation proceeds in the 5′ → 3′ direction, and the transcription bubble travels with the RNA polymerase. The RNA polymerase after imitation of RNA loses the σ factor but continues the polymerisation of ribonucleotides to form RNA.

When the RNA polymerase reaches the terminator region of DNA, the RNA polymerase is separated from DNA-RNA hybrid, it associates transiently with the termination factor (ρ). The nascent RNA separate and the RNA polymerase fall off resulting in termination of transcription.

In prokaryotes, mRNA does not require any processing, so both transcription and translation takes place in cytosol (as there is no separation of nucleus and cytosol in bacteria), Therefore, translation can start much before the mRNA is fully transcribed, i.e., transcription and translation can be coupled.

Transcription in Eukaryotes:

The process of transcription in eukaryotes is similar to that in prokaryotes. Structural genes are monocistronic in eukaryotes.

Two additional complexities are present in eukaryotes:

(i) In the nucleus,, there are at least three RNA polymerases.

(a) RNA Polymerase I Transcribes rRNAs (28 S, 18 S, 5.8 S).

(b) RNA Polymerase II Transcribes precursor of mRNA, which is called heterogeneous nuclear RNA (hnRNA).

(c) RNA Polymerase III Transcribes tRNA, 5 srRNA and (small nuclear RNAs) snRNAs.

(ii) The primary transcript contains both the exons and introns and are non-functional. Therefore, they undergo a process called splicing to remove the introns and to join the exons in the proper order. The process of splicing represents the dominance of RNA world.


Contents

The first fossilized evidence of sexual reproduction in eukaryotes is from the Stenian period, about 1 to 1.2 billion years ago. [10]

Biologists studying evolution propose several explanations for the development of sexual reproduction and its maintenance. These reasons include reducing the likelihood of the accumulation of deleterious mutations, increasing rate of adaptation to changing environments, [11] dealing with competition, DNA repair and masking deleterious mutations. [12] [13] [14] All of these ideas about why sexual reproduction has been maintained are generally supported, but ultimately the size of the population determines if sexual reproduction is entirely beneficial. Larger populations appear to respond more quickly to some of the benefits obtained through sexual reproduction than do smaller population sizes. [15]

Maintenance of sexual reproduction has been explained by theories that work at several levels of selection, though some of these models remain controversial. [ citation needed ] However, newer models presented in recent years suggest a basic advantage for sexual reproduction in slowly reproducing complex organisms.

Sexual reproduction allows these species to exhibit characteristics that depend on the specific environment that they inhabit, and the particular survival strategies that they employ. [16]

In order to sexually reproduce, both males and females need to find a mate. Generally in animals mate choice is made by females while males compete to be chosen. This can lead organisms to extreme efforts in order to reproduce, such as combat and display, or produce extreme features caused by a positive feedback known as a Fisherian runaway. Thus sexual reproduction, as a form of natural selection, has an effect on evolution. Sexual dimorphism is where the basic phenotypic traits vary between males and females of the same species. Dimorphism is found in both sex organs and in secondary sex characteristics, body size, physical strength and morphology, biological ornamentation, behavior and other bodily traits. However, sexual selection is only implied over an extended period of time leading to sexual dimorphism. [17]


Prokaryotic Cell Division

Prokaryotes, such as bacteria, produce daughter cells by binary fission. For unicellular organisms, cell division is the only method to produce new individuals. In both prokaryotic and eukaryotic cells, the outcome of cell reproduction is a pair of daughter cells that are genetically identical to the parent cell. In unicellular organisms, daughter cells are individuals.

To achieve the outcome of cloned offspring, certain steps are essential. The genomic DNA must be replicated and then allocated into the daughter cells the cytoplasmic contents must also be divided to give both new cells the cellular machinery to sustain life. As we’ve seen with bacterial cells, the genome consists of a single, circular DNA chromosome therefore, the process of cell division is simplified. Karyokinesis is unnecessary because there is no true nucleus and thus no need to direct one copy of the multiple chromosomes into each daughter cell. This type of cell division is called binary (prokaryotic) fission.