GENERAL CHARACTERS AND LIFE CYCLE OF BRYOPHYTES

 Bryophytes are the first land plants. It is believed that, they originated from aquatic plant.

Bryophytes are not considered as the successful land plants because vascular tissue is absent and they need water for fertilisation.

Bryophytes are known as amphibians of the plant kingdom, because these plants can live in soil but are dependent on water for fertilisation.

They lack true roots, stem or leaves. They may possess root-like, leaf-like or stem-like structures.

Bryophytes are Sciophytes, i.e. bryophytes prefer to grow in cool, moist (wet) and shady places.

Bryophytes in general are of little economic importance, but some mosses provide food for herbaceous mammals, birds and other animals.

The water conducting tissue in Bryophyta is parenchyma. 

Bryophytes include the various mosses and liverworts that are found commonly growing in moist and shaded areas in the hills.

Life Cycle of Bryophytes : 

• The main plant body of bryophyte is haploid. It produces gametes, hence is called a gametophyte.
• Sex organs are formed on gametophyte.
•  Sex organs are multicellular and jacketed in bryophytes. Male sex organ is called antheridium and female sex organ is called archegonium. An archegonium is flask shaped and produce single egg.
• The male gametes of bryophytes are motile. These motile male gametes are called as antherozoids. Antherozoids are usually comma shaped and biflagellate. Female gamete is called egg or ovum.
• In Bryophyta, fertilization is performed by zoidogamy i.e. male gamete swims into water to reach the female gametes and fertilizes it.
• As a result of the fertilization, a diploid zygote is formed. Zygotes do not undergo reduction division (meiosis) immediately.
• Zygote forms embryo and then sporophyte by mitosis. The zygote initiates the sporophyte generation. Sporophyte generation is a diploid stage.
• The sporophyte is not free-living but attached to the photosynthetic gametophyte and derives nourishment from it. The sporophyte of Bryophyta is not made of root, stem and leaves, but it is made of foot, seta and capsule, so it is known as sporogonium. Some cells present in capsule of sporophyte function as spore mother cells. Now meiosis takes place in spore mother cells. to form haploid spores.
  In Bryophyta the sporophyte is dependent on gametophyte. This is a unique character of Bryophyta.
• The germination of spores is direct or indirect. In liverworts the germination of spore is direct i.e. each spore forms a gametophyte or a thallus after germination. The germination of spores in mosses is indirect. In mosses a multicellular filament is formed after the germination of spore. This filament is known as protonema. Protonema developed from spores is called primary protonema and the protonema developed from parts other than spores are known as secondary protonema.
• Protonema is creeping, green, branched and filamentous. Now lateral buds are formed on protonema. Each bud develops and forms a gametophyte plant. Indirect germination is best for survival. Mosses are gregarious in nature because they appear in group.

LIVERWORTS :  

• Bryophytes included in this class have shape like liver (e.g., Marchantia) or flat (e.g. Riccia) so they are known as liverworts.
• Plant body of this group is thallus like. The thallus is dorsiventral and closely appressed to the substrate.. Rhizoids and scales are present on thallus. Rhizoids are unicellular and unbranched. Scales are multicellular and protective in functions.
• The leafy members (e.g. Porella) have tiny leaf like appendages in two rows on the stem like structures.
• The sporophyte of Liverworts is completely dependent on gametophyte i.e. it is dependent on gametophyte for food, water and habitat.
• The sporophyte of Liverworts is made up of foot, seta and capsule, after meiosis, spores are produced within the capsule. These spores germinate to form free- living gametophytes. True elaters are present in sporophyte of some members of liverworts. Elaters are hygroscopic and they help in dispersal of spores.
• Asexual (vegetative) reproduction in Liverworts takes place by fragmentation of thalli, or by the formation of specialised structures called gemmae (sing. Gemma). Gemmae are green, multicellular, asexual buds, which develop in small receptacles called Gemma cups located on the thalli. The gemmae become detached from the parental body and germinate to form new individuals. e.g. Marchantia.
• During sexual reproduction male and female sex organs are produced either on same (e.g. Riccia) or on different thallus (e.g. Marchantia).

MOSSES :  

• The predominant stage of the life cycle of a moss is the gametophyte which consist of two stages. The first stage is the protonema stage, which develops directly from a spore. It is creeping, green, branched and frequently filamentous stage.
• The second stage is the leafy stage, which develops from the secondary protonema as a lateral bud. They consist of upright, slender axes bearing spirally arranged leaves.
• The main plant body or gametophyte of mosses is made up of stem like, leaf like and rhizoids (roots like). They are attached to the soil through rhizoids. The rhizoids present in the class are multicellular, branched and obliquely septate.
• Vegetative reproduction in mosses is by fragmentation and budding in the secondary protonema.In sexual reproduction, the sex organs antheridia and archegonia are produced at the apex of the leafy shoots. After fertilization, the zygote develops into a sporophyte, consisting of a foot, seta and capsule. The capsule contains spores. Spores are formed after meiosis.
• The sporophyte in mosses is more elaborated (developed) than that in liverworts.
• The sporophyte of mosses is also partially dependent (semi parasite) like that of hornworts. i.e. it is photosynthetic. The Mosses has an elaborate mechanism of spore dispersal.
• Peristomial teeth are present in moss sporophyte which help in spore dispersal.
• Common example of mosses are Funaria, Polytrichum and Sphagnum.

Difference Between Transcription and Replication

 

Replication	Transcription
Occurrence
S phase of the cell cycle	G1 and G2 phase of the cell cycle
Primer
Requires RNA primer for replication to start	Does not require a primer
Enzymes
DNA Polymerase, DNA Helicase	RNA polymerase, Transcriptase
Genome copy
Entire genome is copied	Only certain genes are copied
Position
Found along the DNA strand	Found only along 1 strand of DNA
Raw material
dATP, dTTP, dCTP and dGTP	ATP, GTP, CTP, and UTP
Intend
Conserving genome for further generations	Making copies of RNA of genes individually
Result
Two daughter strands	mRNA, rRNA, non-coding RNA and tRNA
Degradation
Products do not degrade	Products degrade

Difference Between DNA and RNA

Territorial behavior

 A territory is an area held and defended by an organism or group of organisms of the same or different species. Territorial behavior is common to all vertebrates except amphibian but is rare in non-vertebrates.

The exact function of territory formation varies from species to species, but in all cases, it ensures that each mating pair of organisms and their offsprings are adequately spaced to receive a share of the available resources, such as food and breeding space. In this way, species achieves optimum utilization of the habitat.

The size of territories occupied by any particular species varies from season to season according to the availability of environmental resources. Birds of prey and large carnivores have territories several square miles in area in order to provide all their food requirements. Herring gulls and penguins have territories of only a few square metres, since they move out of their territories to feed and use them for breeding purposes only.

Territories are found prior to breeding, usually by males. Defense of the area is greatest at the time of breeding and fiercest between males of the same species. There are a variety of behavioral activities associated with territory formation and they involve threat displays between owners of adjacent territories. These threat displays involve certain stimuli which act as releasers. E.g. An adult male robin would attack another adult male displaying a red breast and a bunch of red feathers, but not a young male robin which did not have a red breast. The level of aggression shown by an organism increases towards the centre of the territory. The aggressiveness of males is determined partly by the level of testosterone in the body and this can affect territory size. E.g. the territory size of a red

grouse can be increased by injecting the bird with testosterone. Territories are acquired through threats, gestures and postures in place of actual fighting. Having obtained a territory, many species especially carnivores proceed to mark out the boundary by leaving a scent trail. This may be done by urinating or rubbing parts of the body against objects called scent posts along the boundary of the territory.

Altruistic behavior

Altruism is a form of social behavior whereby one organism puts itself either at risk or personal disadvantage for the good of other members of the species. In the case of activities associated with and parental care, altruism is not so difficult to comprehend since the action is clearly in the interest of the parents, offsprings and species. E.g. the female baboon protects and cares for its offspring for almost six years whilst most bird species feed and protect their demanding offprings until they are capable of fending for themselves. What is not so clear is the reason why some organisms give support to organisms which are not their offspring E.g birds and monkeys call out warnings to others in danger and female monkeys carry and care for the babies of other monkeys. In insects such as honey bees, wasps and ants, sterile female workers are prevented from producing offsprings, yet they spend their lives looking after their brothers and sisters. Hence, helping their sister (queen) to reproduce, they are effectively aiding in the production of queens, workers an drones with a genetic complement closer to their own than if they had offspring of their own. The conferring of a genetic advantage on closely related organisms forms the basis of altruistic behavior.

Altruistic behavior is very common amongst primates and varies from the extremes of social protection which exist between members of the same troop (monkeys), through acts of mutual grooming and food sharing (apes) to deliberate acts of self-sacrifice for family (God for humans). The extent of altruistic behavior appears to be related to close relatives (kin) such as offspring and siblings (brothers, sisters cousins) with whom they share certain alleles. Thus the adaptive significance of altruistic behavior is to increase the frequency of those alleles common both to the donor and recipient(s) of the altruistic behavior

Pathogenic Properties of Virus

• Viruses have mechanisms to evade host defenses viruses grow inside host cells to hide from immune defense.

• Kill immune cells e.g. HIV – TH Cells.

• Cytopathic effects: - The visible effects of viral infection on host cell. Some effects will kill the cell and some will just change the cells.

• Viruses stop DNA, RNA and/or protein synthesis e.g. Herpes virus block mitosis.

• LySOSomal autolysis of host cells e.g. Influenza: bronchiolar epithelium.

• Production of inclusion bodies (visible viral parts inside the cell) can identify a particular virus e.g. Rabies virus: Negri bodies.

•  Syncytium formation (neighboring cells fuse together) e.g. Varicella Zoster virus.

• Change in cell function e.g. Measles, production of interferons by host cell (triggers host immune response), induce antigenic changes on host cell surface (triggers destruction of infected cell by host immune response).

• Induce chromosomal changes, cell transformation: may activate or deliver oncogenes resulting in loss of contact inhibition (cancer) e.g. Papilloma virus.

Theories on membrane structures

• In 1902 it was thought that the membranes had only lipids (Overton). 
• In 1926 Gorter and Grendell proposed that lipids are capable of forming a double layer.
• In 1935 Danielli and Davson proposed the lipid bilayer model that includes proteins adhering to both lipid-aqueous interfaces.
• Artificial model systems such as the liposomes supported the idea of Danielli and Devson.
• A droplet of lipid made soluble in an organic solvent can be spread over a small hole on a septum that divides two chambers containing water.
• This set up is useful to study biophysical properties of a bilayer such as permeability and electrical resistance.
• Channels for ions can be formed by adding certain proteins or polypeptides.
• Liposomes act as excellent carriers for different molecules such as chemotherapeutic compounds, insulin and antibodies.

Fluid mosaic model

• Fluid mosaic model proposed by S.J. Singer and G.L. Nicolson (1972) was finally acceptable to most biologists.
• This model recognizes that lipids and proteins are in a mosaic arrangement.
• It also recognizes that there is translational movement of lipids and proteins within the lipid bilayer.
• Non covalent interactions ensure a fluid like state for the membranes.
• Integral proteins are intercalated into the continuous lipid bilayer.
• Polar/hydrophilic regions of proteins protrude from the surface while the nonpolar/hydrophobic regions are embedded inside.

Unit membrane model 

• Robertson in 1959 postulated the unit membrane model.
• This model stated that the central layer of plasma membranes is made up of hydrocarbon chains of lipids and the proteins constitute the dense surrounding layers on both sides when viewed through an electronmicroscope.
• Unit membrane model turned out to be an over simplification model as it can’t account for the number of protein molecules present across the membranes.