Gene action in actual cells is accomplished in three steps.
1. Replication: The process in which another DNA is created from DNA is called replication. DNA replication occurs in the S phase of the cell cycle.
2. Transcription: The process by which mRNA is produced from DNA is called transcription. Transcription occurs inside the cell nucleus.
3. Translation: The process by which protein is produced from mRNA is called translation. Translation occurs in the cytoplasm of the cell.

The unit of gene expression is called an operon. Jacob and Mond (1961) proposed the operon model. According to them, an operon is formed by the combination of several genes and their interaction in primitive cells results in the expression of functional genes. They discovered the lactose operon. The operon consists of four segments.
1. Structural genes: It synthesizes enzymes.
2. Driver Gene: It controls the production of protein.
3. Regulatory Gene: A gene that controls the functions of an operon is called a regulatory gene. An increase in the transcription of a gene in a cell is called induction and a decrease is called repression.
4. Promoter or stimulator gene: RNA polymerase enzyme binds to it.
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The word Vertebrata is derived from the Latin word vertebratus meaning having a backbone. Animals in which the embryonic notochord is fully replaced by bony or cartilaginous vertebrae are called Vertebrata. Vertebrata or vertebrates are known as higher order animals. Because their brain is located in the cranium, their other name is craniata. Their species number is 66,178 (IUCN, 2014).
Vertebrates (fish) have been in the ocean for over 150 million years. Hands and feet evolved in a group of vertebrates about 365 million years ago. As a result, they qualify as amphibians to live in water and on land. Over time, the descendants of amphibians adapted and evolved into reptiles, birds and mammals.

Characteristics of Vertebrata
1. Their notochord is replaced by a spinal cord.
2. The anterior part of the hollow nerve forms the brain and the posterior part the spinal cord.
3. They have 5-15 pairs of pharyngeal florets.
4. Lateral pairs of fins or feet act as their locomotion organs.
5. Blood circulatory system is closed type. It is composed of a muscular heart, blood vessels and red blood (containing hemoglobin).
6. Body skin consists of the epidermis (derived from the ectoderm) and the dermis (derived from the mesoderm).
7. 2-4 chambered heart and blood vessels.
8. The brain is protected by the cortex. Brain contains 10-12 pairs of carotid nerves.
9. There is an endoskeleton made of bone or cartilage. Arises from the intraskeletal neural crest.
10. Skin glands, feathers, scales, nails, claws, hairs, horns, hooves etc. exist in the body.
11. There are different types of endocrine glands in the body.
12. Nutrition is complete; Consists of muscular alimentary canal, liver and pancreas.
13. There are well-developed kidneys for excretion and osmoregulation.
[Key words to remember features: spinal cord, brain, mesentery, pharynx, feet, fins, closed circulatory system, carotid nerve, endoskeleton, excretion, osmoregulation]

1. Their notochord is replaced by a spinal cord.
2. The anterior part of the hollow nerve forms the brain and the posterior part the spinal cord.
3. They have 5-15 pairs of pharyngeal florets.
4. Lateral pairs of fins or feet act as their locomotion organs.
5. Blood circulatory system is closed type. It is composed of a muscular heart, blood vessels and red blood (containing hemoglobin).
6. Body skin consists of the epidermis (derived from the ectoderm) and the dermis (derived from the mesoderm).
7. 2-4 chambered heart and blood vessels.
8. The brain is protected by the cortex. Brain contains 10-12 pairs of carotid nerves.
9. There is an endoskeleton made of bone or cartilage. Arises from the intraskeletal neural crest.
10. Skin glands, feathers, scales, nails, claws, hairs, horns, hooves etc. exist in the body.
11. There are different types of endocrine glands in the body.
12. Nutrition is complete; Consists of muscular alimentary canal, liver and pancreas.
13. There are well-developed kidneys for excretion and osmoregulation.
[Key words to remember features: spinal cord, brain, mesentery, pharynx, feet, fins, closed circulatory system, carotid nerve, endoskeleton, excretion, osmoregulation]

The word Vertebrata is derived from the Latin word vertebratus meaning having a backbone. Animals in which the embryonic notochord is fully replaced by bony or cartilaginous vertebrae are called Vertebrata. Vertebrata or vertebrates are known as higher order animals. Because their brain is located in the cranium, their other name is craniata. Their species number is 66,178 (IUCN, 2014).
Vertebrates (fish) have been in the ocean for over 150 million years. Hands and feet evolved in a group of vertebrates about 365 million years ago. As a result, they qualify as amphibians to live in water and on land. Over time, the descendants of amphibians adapted and evolved into reptiles, birds and mammals.

A pair of mucus-secreting folds in the pharynx of urochordate and cephalochordate animals is called the endostyle. Endostyle is a parallel longitudinal fold. It is located at the base of the pharynx. In vertebrates, the endostyle transforms into a thyroid gland. For example – Branchiostoma.

The oral hood of cephalochordate animals has narrow thread-like structures called oral cirri. Oral cirri hang around the mouth. It is a sensory organ. It acts as a filter. Such as Branchiostoma

In cephalochordates, the boat-like structure around the mouth is called the oral hood. The facial metapleural fold expands to form the oral hood. For example, Branchiostoma.

The word Cephalochordata is formed from the Greek word Kephale meaning head and chorda meaning chord. They are commonly called lancelets. First they are described as Amphioxus (amphi = both means both ends, ends = oxys means sharp). Later, according to the law of precedence, they were named Branchiostomata (branchia = gill, stoma = mouth). Cephalochordates are called invertebrate chordates. In 1774 scientist Peter Simon Pallas (1774) described Branchiostoma lanceolatum. Their number of species is 33.
Characteristics of Cephalochordata
1. Body long, slender, laterally compressed, transparent, boat-shaped and pointed at both ends.
2. Anterior to the body is an oral hood and oral cirri.
3. From the front to the back of the body there is a permanent notochord and nerve cord.
4. The pharynx has numerous gill openings. The gills open into the atrium.
5. At the tip of the body are atriopores.
6. The body is 3-7 cm long and has 60 pairs of myotome muscles on both sides of the body.
7. They look a lot like Ballam Aga. That is why they are nicknamed Bhallaka animals.
8. Their pharynx contains numerous atria and atriopores.
9. They are excreted.
10. They are called lancelets in English.
11. They do not have a clear head.
12. The circulatory system is closed and developed. Hepatic portal system is present.
13. The main excretory organ is protonephridia. Protonephridia contain solenocytes cells.
[KEY WORDS TO REMEMBER CHARACTERISTICS: boat, oral hood, oral cirri, notochord, nerve cord, pharynx, myotome muscle, excreta, lanceolate, protonephridia]

Some animals of Cephalochordata
Branchiostoma lanceolatum (Amphioxus)
Branchiostoma belcheri
Branchiostoma floridae
Asymmetron inferum.

Branchiostoma lanceolatum (Amphioxus)
Branchiostoma belcheri
Branchiostoma floridae
Asymmetron inferum.