Most Important Diagrams Class 12 Biology (Zoology) | NEET 2023 Biology Exam Preparation

Diagrams always make it easy for students to understand the topic. Some of the important class 12 Zoology diagrams are discussed in this article in perspective of the upcoming NEET exam. Questions based on diagrams have become frequent in recent exams. Thus, a good understanding of the diagram concepts will prove beneficial while attempting the Zoology/Botany section. Here, diagrams from chapters like reproductive system, evolution and biotechnology are covered.

Table of Contents:

• Human Reproduction
• Reproductive Health
• Evolution
• Human Health and Diseases
• Biotechnology: Principles and Processes
• Biotechnology and its Applications

Human Reproduction

The reproductive system ensures the continuation of species. Normally, most animals are either definite females or definite males. The testes in the males produce sperm (male gamete) and the ovaries in the females produce ovum or egg (female gamete). The diagrams representing the male and female reproductive systems and the male and female gamete formation are discussed here.

Male Reproductive System

Testes are the primary sex organs in males. The accessory sex organs include seminal vesicles, prostate gland, urethra and penis. In these, the external genital organs are the penis, scrotum and urethra. The remaining constitute the internal genitalia.

The testes (a pair) are suspended in a sac-like pouch called the scrotum. Each testis is composed of coiled tubes known as the seminiferous tubules. This seminiferous tubule continues as vas efferens which forms epididymis and continues as vas deferens. The seminiferous tubule of testis opens into vas efferens via the rete testis. The rete testis, vas efferens, epididymis and vas deferens form the accessory male sex ducts.

The vas deferens receive ducts from the seminal vesicle and open into the urethra as ejaculatory ducts. Urethra extends through the penis to its opening called the urethral meatus.

Sperm

Sperm consists of 4 parts – head, neck, middle piece and tail. It is the only cell in the body to contain flagella. The anterior two-thirds of the head is covered by a cap-like structure called the acrosome. It has enzymes essential for the sperm to fertilize the ovum. The head is connected to the middle piece by a short neck. This middle piece (body) has mitochondria that provide energy for sperm movement. The tail is a flagellum that protrudes out of the cell body and aids in the movement of the sperm.

Female Reproductive System

The primary sex organs in females are the ovaries that produce gametes (ova/egg). The secondary sex organs include a system of genital ducts like the fallopian tube, cervix, uterus and vagina and also the external genitalia like the pubis, labia majora, labia minora, clitoris and hymen.

The fallopian tube (oviduct) or salping comprises 3 parts – the infundibulum (part closer to ovary), ampulla (wider part of the oviduct) and the isthmus (last part of the oviduct) that joins with uterus. Fimbriae are finger-like projections on the edges of the infundibulum that collects ovum after ovulation.

The uterus is a pear-shaped structure made of 3 layers – outer thin perimetrium, middle thick myometrium and inner glandular endometrium. The uterus opens into the vagina via a narrow cervix. Ovaries are the primary sex organs that are connected to the pelvic wall and uterus by ligaments. The structure of the ovary is discussed below with a diagram.

Sectional View of Ovary

Ovaries are flattened ovoid bodies, with dimensions of 2 to 4 cm in length and are attached to the uterus and pelvic wall by ovarian ligaments. Each ovary has two portions – medulla and cortex. The cortex consists of ovarian follicles, connective tissue and interstitial cells. The central stroma of loose connective tissue is the medulla.

The process of formation of mature female gamete is called oogenesis. The Graafian follicle is the final mature follicle that undergoes ovulation. Thus the functions of the ovaries are – the release of female sex hormones, oogenesis and regulation of the menstrual cycle.

Spermatogenesis

The immature male germ cells produce sperm by spermatogenesis. The spermatogonia in the seminiferous tubules multiply by mitotic division and increase in number. Each spermatagonium contains 46 chromosomes and is diploid. They divide by mitosis without any change in their chromosome number. The last mitotic generation enters the stage of growth as primary spermatocytes which undergo meiosis. This can further be divided into 2 growth phases.

In the first phase, the primary spermatocyte divides into 2 secondary spermatocytes. They receive only the haploid or half the number of chromosomes. During the second phase, each secondary spermatocyte undergoes 2nd meiotic division, resulting in two smaller spermatids. Each spermatid has a haploid number of chromosomes and there is no further division. These spermatids are transformed into mature sperm cells.

Seminiferous Tubule

As mentioned above, the process of transformation of spermatids to sperm is called spermiogenesis. After spermiogenesis, the sperm’s head is embedded in the Sertoli cells or supporting cells of the seminiferous tubule by the process called spermination. This process starts at the age of puberty due to the increased secretion of GnRH (gonadotropin-releasing hormone).

The diagrammatic sectional view of the seminiferous tubule is also important for the exam.

Oogenesis

The process of formation of a mature female gamete is called oogenesis. It is initiated during embryonic development when millions of oogonia are formed within each fetal ovary. This process starts with a germ cell called oogonia which undergoes mitosis to increase in number.

Oogonia start dividing and enter prophase-Ⅰ of meiosis and get temporarily arrested as primary oocytes. Each primary oocyte gets surrounded by primary follicles which further get surrounded by more layers to form secondary follicles and then the tertiary follicles. The primary oocyte within the tertiary follicles completes its first meiotic division to form large haploid secondary oocytes and a tiny first polar body.

The tertiary follicle further changes into a mature Graafian follicle. Now the secondary oocyte forms a zona pellucida layer surrounding the Graafian follicle and it ruptures to release the ovum by a process called ovulation.

Menstrual cycle

During each menstrual cycle, a series of changes occur in the ovary and accessory sex organs. These changes are divided into 4 groups – ovarian changes, uterine changes, vaginal changes and the changes in the cervix. These changes can be divided into the following phases –

1. Menstrual phase – The cycle starts with this phase by the shedding of the uterus lining and it lasts for 3 to 5 days
2. Follicular phase – Here, the primary follicle grows into a mature Graafian follicle and the endometrium regenerates through proliferation.
3. Ovulatory phase – The mid-phase (13-17 days) where the LH and FSH hormones attain their peak. The LH surge ruptures the Graafian follicle and leads to the release of ovum.
4. Luteal phase – The remains of the Graafian follicle form the corpus luteum. The regressed form of corpus luteum is corpus Albicans.

The diagram represents ovarian, hormonal and uterine events that happen during menstruation. Menstruation is repeated at an average interval of 28/29 days and the cycle of events from one menstruation to the next is termed the menstrual cycle.

Mammary Gland

Mammary glands are paired structures that contain a variable amount of fat and glandular tissue. The glandular tissues of each gland have 15-20 mammary lobes which comprise clusters of cells called alveoli. The alveoli are lined with milk-secreting epithelial cells which further open into the mammary tubules. The tubules of each mammary lobe join to form a mammary duct. Likewise, many mammary ducts join to form the ampulla which is connected to the lactiferous duct via which milk is sucked out.

Reproductive Health

The most important step to check population growth is by using various contraceptive methods. They are birth control methods that also prevent unwanted pregnancies. Some common contraceptive devices are – diaphragm, Intrauterine devices (IUD), condoms and implants. Other methods include oral contraceptives, surgical methods of contraception, etc.

1. Condoms are barrier contraceptives that are made of latex or rubber sheath. Male condom covers the penis and the female condom (femidom) covers the cervix or vagina during copulation.
2. Dome-shaped diaphragms or cervical caps or vaults can also be used as barriers. They have higher efficiency due to the usage of spermicide in them.
3. The IUDs (intrauterine devices) are inserted in the female reproductive tract to prevent pregnancy. They can be classified into non-medicated IUDs (Lippes loop), copper-releasing IUDs (CuT, Cu7, Multiload 375) and hormonal IUDs (LNG-20 and progestasert).
4. The surgical methods include vasectomy (male sterilisation) and tubectomy (female sterilisation). Vasectomy includes tying up or cutting and removal of vas deferens. Likewise, tubectomy includes tying or removal of the fallopian tube.

Evolution

This chapter deals with quite a lot of diagrams that could be asked in NEET. It deals with the origin of life, adaptive radiation, mechanism of evolution and evolution of mankind. The chapter also covers some schematic representations. Some of the most important diagrams are as follows.

Urey-Miller Experiment

Urey and Miller did an experiment to prove how life forms could have emerged from non-living organic molecules like proteins, RNA, etc. They designed a spark discharge apparatus to perform a simulation experiment. The experimental model contained a half-filled flask with water and another flask with a pair of electrodes and gases like methane, ammonia, hydrogen (in the ratio of 2:1:2 respectively) and water vapour to imitate the primitive earth’s atmosphere. They created electric discharge in the flask containing these gases at 800°C and observed the formation of amino acids like glycine, alanine and aspartic acid.

Evolution of Plants and Animals through Ages

The evolutionary sequence of Palaeozoic, Mesozoic and Cenozoic are to be remembered along with the significant happenings during each period.

Operation of Natural Selection on Different Traits

Natural Selection is a process by which physically, physiologically and behaviourally well-adapted organisms survive and reproduce. The three types of selection that can occur are:

1. Stabilising selection
2. Directional selection
3. Disruptive selection

When two extreme traits are competing and the natural selection favours an intermediate trait then it is called the stabilising selection. When the selection process favours one extreme trait then it is called directional selection. Disruptive selection is an uncommon process where both extreme traits are favoured against the intermediate one.

Adaptive Radiation of Marsupials of Australia

Similar to the adaptive radiation in Darwin’s finches, there is another example given in NCERT called the adaptive radiation of marsupials in Australia. The diagram represents various marsupials like Kangaroo, Tasmanian wolf, Koala, Bandicoot, etc., arising from an ancestral stock but within the Australian island. The Australian placental mammals also exhibit similar adaptive radiation.

Also Check: NEET Flashcards: Evolution

Human Health and Diseases

This chapter contains some of the most important diagrams of the class 12 syllabus. The chapter discusses infectious and non-infectious diseases and also the concept of immunity and vaccination.

Life cycle of plasmodium

The life cycle of plasmodium involves 2 hosts. First the development happens in the blood-feeding insect (Anopheles mosquito) which later injects the plasmodium in the form of infectious sporozoites into a vertebrate host (humans). The parasite often grows within the liver tissue before making its way through the bloodstream. There they attack and rupture the red blood cells which are associated with the release of haemozoin (toxic substance). This will result in high fever and chills. When a female Anopheles bites an infected person, the parasite grows and gets stored in its salivary gland as sporozoites. These sporozoites can be injected into another human host and the cycle continues.

Antibody Structure

Each antibody has 4 peptide chains – two long heavy chains and two small light chains. These chains are bound together by disulfide bonds to form a Y shape and the structure is represented as H₂L₂. Both heavy and light chains have variable and constant regions. The structure also comprises the antigen-binding fragment (Fab) present on the N terminus and the crystallisable fragment (Fc) on the C terminus of the chain.

The antibodies or immunoglobulins (Ig) produce IgA, IgE, IgM and IgG in our body.

Replication of Retrovirus

Retrovirus (Eg., HIV – Human Immunodeficiency Virus) is an enveloped structure that includes the RNA genome. Once a retrovirus infects a normal cell, the viral RNA is introduced into the host cell. This undergoes reverse transcription and thus results in viral DNA. Now, the viral DNA is introduced into the host cell’s DNA, where it starts replicating.

The viral DNA is transcribed and translated, which further results in new viruses that can infect other cells. The sequential step is well explained in the diagram given below.

Biotechnology: Principles and Processes

This chapter talks about various advanced biotechnological techniques and explains the concept of recombinant DNA technology and cloning vectors through simple diagrams. Also, the following chapter discusses the application of biotechnology.

Recombinant DNA technology

This technique involves the selection of the desired gene which needs to be inserted into the host cell. The process also involves a suitable cloning vector in which the desired gene has to be integrated. Thus, the host consumes the vector with desired DNA and becomes the final tool of recombinant DNA technology. The sequential process of rDNA technology is described below in the diagram.

E.coli Cloning Vector pBR322

We talked about cloning vectors in the previous concept. These vectors are small DNA fragments that can easily replicate and integrate with the host. Plasmids are also a type of cloning vector. Here, let us look at the E.coli cloning vector pBR322 and its characteristic features.

• It has Ori or the origin of replication from where the replication begins.
• It has restriction sites like BamH Ⅰ, EcoR Ⅰ, Sal Ⅰ, Cla Ⅰ, Pvu Ⅰ, Pst Ⅰ, Pvu Ⅱ and Hind Ⅲ.
• It contains selectable markers like antibiotic resistance genes for tetracycline (tet^R) and ampicillin (amp^R)

Biotechnology and its Applications

The biotechnological techniques can be used in agriculture, farming, medicine and research.

PCR – Polymerase Chain Reaction

In the PCR technique, multiple copies of the desired gene or DNA are synthesised in vitro using DNA polymerase and 2 sets of primers (oligonucleotide primers). The following are 3 important steps in the polymerase chain reaction which are also represented in the diagram.

1. The double-stranded DNA is turned into single-stranded DNA by the process of denaturation. Here, the hydrogen bonds are broken at a high temperature.
2. Then the oligonucleotide primers undergo annealing at an optimum temperature (40 – 65°C). Here, the primers cohere with the complementary sequence on the DNA template.
3. In the elongation process, the DNA polymerase extends the primers using genomic DNA and the nucleotides provided in the reaction as templates. This process of continuous amplification is better achieved by a thermostable DNA polymerase called the Taq polymerase.

Finally, the amplified DNA fragment can be ligated with a vector to proceed with further cloning.

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