CBSE Class 12 Biology — NCERT Solutions

The male gametophyte develops inside the microsporangium of the anther (part of the stamen), where pollen grains are formed. The female gametophyte develops inside the ovule (megasporangium) present in the ovary.

Microsporogenesis is the formation of microspores from the pollen mother cell (PMC) inside the anther. Megasporogenesis is the formation of megaspores from the megaspore mother cell (MMC) inside the ovule.

In both cases, the cell division is meiosis.

At the end of microsporogenesis, a microspore tetrad is formed, and each microspore later develops into a pollen grain.

At the end of megasporogenesis, four megaspores are formed, out of which usually one functional megaspore develops into the embryo sac (female gametophyte).

Humans are sexually reproducing, as stated in the chapter.

Humans give birth to live young, so they are viviparous.

Reproductive health is significant because it means a society with people having physically and functionally normal reproductive organs and normal emotional and behavioural interactions in all sex-related aspects. It helps in maintaining healthy families, reducing maternal and infant mortality, controlling population growth, preventing STDs, and improving overall well-being of the society.

The aspects needing special attention are: adolescent reproductive health, proper information about reproductive organs and changes during adolescence, safe and hygienic sexual practices, prevention and treatment of STDs and AIDS, birth control, care of pregnant and post-natal mothers, breast-feeding, small family norms, and prevention of sex abuse and other sex-related crimes.

Advantages of pea plant for Mendel’s experiments:

• It had many clear contrasting traits such as tall/dwarf, round/wrinkled, yellow/green, etc.
• It was a true-breeding plant, so stable traits were available for experiments.
• It is naturally self-pollinating, but can also be cross-pollinated easily by Mendel.
• It had a short life cycle, so successive generations could be studied quickly.
• It produced a large number of offspring, giving reliable statistical results.

A nitrogenous base is the base part only, while a nucleoside is a base linked to a sugar. From the list:

• Bases: Adenine, Thymine, Uracil, Cytosine
• Nucleosides: Cytidine, Guanosine

Antibiotic resistance in bacteria is explained by Darwinian natural selection. In a bacterial population, there is variation: some bacteria may already have mutations that make them less affected by an antibiotic. When the antibiotic is used, the non-resistant bacteria are killed, but the resistant ones survive because they are better fit in that environment. These survivors then reproduce rapidly and pass the resistance to their offspring. Over time, the population becomes mostly resistant. This shows that heritable variations that improve survival are selected by nature.

Public health measures to safeguard against infectious diseases include:

• Proper disposal of waste and excreta
• Periodic cleaning and disinfection of water reservoirs, pools, cesspools and tanks
• Observing standard hygiene practices in public catering
• For air-borne diseases, avoiding close contact with infected persons or their belongings
• For vector-borne diseases, controlling or eliminating vectors and their breeding places by:
• avoiding stagnation of water around homes
• regular cleaning of coolers
• using mosquito nets
• introducing fishes like Gambusia in ponds
• spraying insecticides in ditches, drainage areas and swamps
• providing wire mesh on doors and windows
• Maintaining personal hygiene and clean food and water
• Vaccination/immunisation against infectious diseases

I would carry a sample of curd from home because it contains lactic acid bacteria (LAB) in very large numbers. These microbes can be easily seen under a microscope, and curd is a common household sample in which microbes are present.

DNA is bigger in molecular size than enzymes. We know this because DNA is made up of very long chains of nucleotides, whereas enzymes are proteins made of amino acids and are comparatively smaller molecules. In biotechnology, DNA fragments are handled, cut, and separated, but enzymes such as restriction endonucleases and ligases act on them, showing that DNA is the larger substrate.

The meristematic region is best suited because it is generally free of virus. Even if the plant is infected, the virus usually does not enter the meristem, so by removing the meristem and growing it in vitro, one can obtain virus-free plants.

Populations possess the following attributes that individual organisms do not:

• Birth rate and death rate (as per capita rates)
• Sex ratio
• Age distribution
• Population density / size

An individual may have births and deaths, but only a population has these measurable collective properties.

Plants are called producers because they fix carbon dioxide and synthesize organic food by photosynthesis.

The three important components of biodiversity are:

1. Genetic diversity
2. Species diversity
3. Ecological diversity