Meiosis Explained

It is a type of cell division that occurs only in the reproductive cells and hence it is known as germ cell division. The daughter cells resulting from meiosis will carry half the number of chromosomes and half the amount of genetic material compared to the parent cell. Hence, meiosis is also known as reductional division.

fig. 17.5 - Stages of Meiosis

Meiosis occurs during the formation of gametes (spermatozoa and ova) in animals and spores in higher plants. It is mainly meant for bringing about a decrease in the chromosome number from the diploid (2n) condition to the haploid (n) condition.

The most characteristic feature in meiosis is that nucleus divides twice successively. Hence, meiosis is conventionally divided into first meiotic division (or meiosis - I) and second meiotic division (or meiosis - II).

In both the meiotic divisions, karyokinesis can be distinguished into four stages - prophase, metaphase, anaphase and telophase.

First Meiotic Division

The first meiotic division or meiosisI is commonly described as reductional division since at the end of this division the two resulting daughter cells will have half the number of chromosomes as that of the parent cell.

Interphase

It is the preparatory phase. Cell organells replicate and size of the cell increases. DNA molecule undergoes replication. Each chromosome exists as a pair of chromatids joined together by a centromere.

fig. 17.6 Stages of Meiosis-I

Prophase I

It is the phase of longest duration and involves a series of significant changes in the chromosomes. These changes are often described in five substages namely leptontene, zygotene, pachytene, diplotene and diakinesis.

Leptotene

• : Chromosomes shorten and become visible as single structures. In some cases they have a beaded appearance showing densely staining material called chromomeres alternating with nonstaining regions.

Zygotene

• : Paternal and maternal chromosomes come together and pair up. This pairing of homologous chromosmes is called synapsis. The paired chromosomes are described as bivalents. The bivalents shorten and thicken (spiralisation).

Pachytene

• : Each chromosome splits into two chromatids and thus each pair will have four chromatids two paternal and two maternal. They are now called tetrads. The non-sister chromatids of the paternal and maternal chromosomes overlap each other. They appear to be joined at several regions along their length. These points are called Chiasmata. Each chiasma is the site of an exchange of genetic material between the two chromatids. It occurs due to breakage and reunion between the two non-sister chromatids. This process is called genetic recombination.

Diplotene

• : The synaptic forces holding the two chromosomes in the pair come to an end. The chromosomes start separating. This separation is called as disjunction.

Diakinesis

• : Separation of the chromosomes is now complete with paternal and maternal chromosomes having exchanged portions of chromatids. The chromosomes condense again. The chiasmata disappear by sliding towards the tips of the chromatids. This process is called terminalisation.

By the time these changes are completed in the chromosomes, the nuclear membrane and nucleolus disappear. Asters and spindle fibres make their appearance.

fig. 17.7 - Behaviour of Chromosomes during Meiosis-I

Metaphase-I

In this change of a very brief duration, the chromosomes move towards the equator of the cell and come to lie in two parallel metaphase plates. These two parallel plates are formed by one set each of the homologous chromosomes. Each homologous chromosome has two kinetochores, one for each of its two chromatids.

Anaphase-I

There is no splitting of the centromere. As a result homologous chromosomes of each pair rather than the chromatids of a chromosome separate and move to the opposite poles. As a result, half the number of chromosomes that appear in the early prophase, move to each opposite pole. It is here that an actual reduction in the chromosome number (from (2n) to (n)) occurs. However, each chromosome found at the poles consists of two chromatids.

This is in contrast to the single stranded chromosomes in the anaphase of mitosis.

Telophase-I

The chromosomes at each pole uncoil and elongate to form the chromatin. A nucleolus reappears at each pole. Spindle fibres and asters disappear and centrioles split. A nuclear membrane is formed at each pole resulting in the formation of two daughter nuclei.

Cytokinesis - I

Simultaneously with the formation of two daughter nuclei, a cleavage furrow appears in the middle of the cell. The furrows gradually deepen and divide the cell into two daughter cells. Each of the resulting daughter cell prepares itself to undergo the second meiotic division.

Interkinesis

There is no interphase preceding second meiotic division. There is a brief intervening period called interkinesis. During this period there may be synthesis of some reserve food and proteins. However, there is no replication of DNA prior to meiosis II.

Second Meiotic Division

The second meiotic division or meiosis II almost always follows the first meiotic division. This division is primarily meant for separating the two chromatids of each chromosomes. Meiosis-II also has a Karyokinesis and a cytokinesis.

Karyokinesis of meiosis-II (Karyokinesis-II) can be distinguished into four stages namely prophase-II, metaphase-II, anaphase-II and telophase-II.

Prophase-II

It is of a short duration compared to prophase-I. No significant changes take place in the chromosomes. Nuclear membrane and nucleolus disappear. Asters and spindle fibres are formed.

Metaphase-II

The chromosomes line up at the equator of the cell forming a single metaphase plate (as in mitosis).

fig. 17.8 - Stages of Meiosis-II

Anaphase-II

The centromere splits and two chromatids in each chromosome start moving away from each other. Finally, they reach the poles of the cell. Each pole now has haploid number of chromosomes and half the amount of DNA.

Telophase-II

Chromosomes at each pole uncoil and elongate to form chromatin. Nucleolus and nuclear membrane are formed surrounding each chromatin network. Asters and spindle fibres disappear and centrioles divide. Daughter nuclei are formed.

Cytokinesis-II

A cleavage furrow appears in the middle of the cell, deepens gradually and divides the cell into two.

Thus, at the end of second meiotic division, four daughter cells are formed. Each daughter cell has not only half the number of chromosomes but also half the amount of DNA, as that of the parent cell. Thus, the resulting cells are truly haploid cells.

Significance of Meiosis

Meiosis becomes significant for the following reasons.

• It brings about a reduction in the chromosome number from a diploid (2n) condition to a haploid (n) condition. Such a reduction becomes necessary for maintaining the chromosome number.

• It provides chance for the appearance of new gene combinations as a result of crossing over. This situation brings about variations.

• It is a division necessary for the formation of gametes in animals and spores in plants.

Both mitosis and meiosis essentially follow the same sequence in all living organisms, which is an evidence of the basic relationship between diverse groups of living organisms

Mitosis Explained

It is a common type of cell division that occurs in all the cells of an organism. Hence, it is commonly called as somatic cell division. In mitosis, the resulting daughter cells will have the same number of chromosomes and contain the same amount of DNA, as that of the parent cell. Hence, mitosis is commonly described as equational division.

Mitosis occurs in two stages namely karyokinesis, the division of nucleus and cytokinesis, the division of cytoplasm. Just prior to karyokinesis, the cell will be in interphase.

fig. 17.2 - Stages of Mitosis

Interphase

It is the preparing phase. It is of varying duration depending on the cell type function. It is the period in which the cell carries out synthesis of organelles and increases in size. The nucleoli are prominent and actively synthesising ribosomes. Just prior to division, the DNA undergoes replication. Each chromosome exists as a pair of chromatids joined together by a centromere.

Karyokinesis

It is the division of nuclear material, represented by a sequence of events in the cell. It can be distinguished into four phases namely prophase, metaphase, anaphase and telophase.

Prophase

It is the longest stage of the division cycle. It is characterised by significant changes.

• Chromatids shorten (to about 4% of their original length) and thicken by spiralisation and condensation of DNA

• Centrioles move to the opposite poles of the cell

• Short microtubules develop, radiating from the centrioles. These are called asters

• Nucleolus gradually decreases in size and disappears

• Nuclear membrane disintegrates

• Spindle fibres appear in the cytoplasm

Metaphase

In this phase, chromosomes move to the equator of the cell.

• Pairs of chromatids become attached to the spindle fibres at their centromeres

fig. 17.3 - Stages of Mitosis

Anaphase

It is a rapid stage.

• Each centromere splits into two

• Spindle fibres pull the daughter centromeres to the opposite poles

• The separated chromatids, now called chromosomes, are pulled along with centromeres to the opposite poles

Telophase

It is the last phase of Karyokinesis.

• Chromosomes reach the poles of the cell, uncoil and lengthen to form chromatin

• Spindle fibres disintegrate and centrioles replicate

• A nuclear membrane is formed around chromosomes in each pole

• Two daughter nuclei are formed

As telophase is in progress, cytokinesis begins in the cell.

Cytokinesis

It is the division of cytoplasm. It occurs in animal cells by the appearance of a furrow in the middle of the cell. The furrow deepens and divides the cell into two. Two daughter cells are formed.

fig. 17.4 - Differences between Mitosis in Plant and Animal cells

Significance of Mitosis

Mitosis becomes significant for the following reasons.

• Mitosis forms two daughter cells which will have the same chromosome number and same genetic material as the parent cell.

• Daughter cells formed from mitosis are genetically identical to their parent cell and no variation would be introduced during mitosis. This results in genetic stability within the populations of cells derived from parental cells, as in a clone.

• The number of cells within an organism increases by mitosis and this process is called hyperplasia. It forms the basis for growth.

If mitotic division goes uncontrolled in any part of the body, it results in the formation of malignant cells. These cells continue to divide resulting in the formation of malignant tumours. This condition is called cancer.

• Mitosis is the basis of asexual reproduction in both plants and animals. This becomes the basis for vegetative propagation.

• Mitosis is also responsible for repair and regeneration of the injured and lost parts of the body.

Intro to the Cell Cycle

Every cell that is capable of undergoing division passes through a cyclic sequence of events involving growth and division. It is called Cell Cycle. It encompasses the entire sequence of events that occur in a cell from the time it is formed from its parent cell till the time of its own division into daughter cells.

Cell cycle has three main stages namely:

Interphase

This is a period of intense synthesis and growth in the cell. The cell produces many materials required for its own growth and activities. The genetic material DNA replicates during interphase.

Karyokinesis

It is the process of nuclear division, which involves separation of chromatids and their redistribution as chromosomes into daughter cells.

Cytokinesis

It is the process of division of the cytoplasm to result in the formation of daughter cells.

fig. 17.1 - The Cell Cycle

Phase	Events within cell
G1	Intensive cellular synthesis, mitochondria, chloroplasts (in plants), ER, lysosomes, golgi complex, vacuoles and vesicles produced. Nucleus produces rRNA, mRNA and tRNA and ribosomes are synthesised. Cell produces structural and functional proteins. Cell metabolic rate high and controlled by enzymes. Cell growth occurs. Substances produced to inhibit or stimulate onset of next phase.
S	DNA replication occurs. Protein molecules called histones are synthesised and cover each DNA strand, Each chromosome has become two chromatids.
G2	Intensive cellular synthesis. Mitochondria and chloroplasts divide. Energy stores increase. Mitotic spindle begins to form.
Mitosis	Nuclear division occurs in four phases
C	Equal distribution of organelles and cytoplasm into each daughter cells

The length of the cycle depends on the nature of cell and various external factors like temperature food and oxygen availability. Bacterial cells may divide every 20 minutes, epithelial cells living the small intestine divide once in 8 to 10 hours, onion root tip cells take about 20 hours to divide. Some specialised cells like the nerve cells never divide.

Embyrology

Tissue and Cell Types

Overview of Meiosis

Overview of Meiosis

Meiosis is a two-part cell division process in organisms that sexually reproduce. Meiosis produces gametes with one half the number of chromosomes as the parent cell.

In some respects, meiosis is very similar to the process of mitosis, yet it is also fundamentally different.

The two stages of meiosis are meiosis I and meiosis II. At the end of the meiotic process, four daughter cells are produced. Each of the resulting daughter cells has one half of the number of chromosomes as the parent cell.

Before a dividing cell enters meiosis, it undergoes a period of growth called interphase.

During interphase the cell increases in mass, synthesizes DNA and proteins, and duplicates its chromosomes in preparation for cell division.

Meiosis I

Meiosis I encompasses four stages:

• Prophase I
  
• Metaphase I
  
• Anaphase I
  
• Telophase I
  

In most cases, at the end of meiosis I, two daughter cells are produced.

Meiosis II

Meiosis II encompasses four stages:

• Prophase II
  
• Metaphase II
  
• Anaphase II
  
• Telophase II
  

At the end of meiosis II, four daughter cells are produced. Each of these resulting daughter cells is haploid 

Before a dividing cell enters meiosis, it undergoes a period of growth called interphase.

Interphase:

• G1 phase: The period prior to the synthesis of DNA. In this phase, the cell increases in mass in preparation for cell division. Note that the G in G1 represents gap and the 1 represents first, so the G1 phase is the first gap phase.
• S phase: The period during which DNA is synthesized. In most cells, there is a narrow window of time during which DNA is synthesized. Note that the S represents synthesis.
• G2 phase: The period after DNA synthesis has occurred but prior to the start of prophase. The cell synthesizes proteins and continues to increase in size. Note that the G in G2 represents gap and the 2 represents second, so the G2 phase is the second gap phase.
• In the latter part of interphase, the cell still has nucleoli present.
• The nucleus is bounded by a nuclear envelope and the cell's chromosomes have duplicated but are in the form of chromatin.
• In animal cells, two pair of centrioles formed from the replication of one pair are located outside of the nucleus.

Overview of Mitosis

Overview of Mitosis

Cell division is an elegant process that enables organisms to grow and reproduce. Through a sequence of steps, the replicated genetic material in a parent cell is equally distributed to two daughter cells. While there are some subtle differences, mitosis is remarkably similar across organisms.

Before a dividing cell enters mitosis, it undergoes a period of growth called interphase. Interphase is the "holding" stage or the stage between two successive cell divisions. In this stage, the cell replicates its genetic material and organelles in preparation for division.

Mitosis is composed of several stages:

• Prophase
  
• Metaphase
  
• Anaphase
  
• Telophase
  

Let's briefly look at some important events in each step in the process.

Prophase

In prophase, the chromatin condenses into discrete chromosomes. The nuclear envelope breaks down and spindles form at opposite "poles" of the cell.

Metaphase

In metaphase, the chromosomes are aligned at the metaphase plate (a plane that is equally distant from the two spindle poles).

Anaphase

In anaphase, the paired chromosomes (sister chromatids) move to opposite ends of the cell.

Telophase

In this last stage, the chromosomes are cordoned off in distinct new nuclei in the emerging daughter cells. Cytokinesis is also occurring at this time.

At the end of mitosis, two distinct cells with identical genetic material are produced.

Before a dividing cell enters mitosis, it undergoes a period of growth called interphase. Some 90 percent of a cell's time in the normal cellular cycle may be spent in interphase.

View image of a cell in interphase.

Stages of Interphase

• G1 phase: The period prior to the synthesis of DNA. In this phase, the cell increases in mass in preparation for cell division. Note that the G in G1 represents gap and the 1 represents first, so the G1 phase is the first gap phase.
• S phase: The period during which DNA is synthesized. In most cells, there is a narrow window of time during which DNA is synthesized. Note that the S represents synthesis.
• G2 phase: The period after DNA synthesis has occurred but prior to the start of prophase. The cell synthesizes proteins and continues to increase in size. Note that the G in G2 represents gap and the 2 represents second, so the G2 phase is the second gap phase.
• In the latter part of interphase, the cell still has nucleoli present.
• The nucleus is bounded by a nuclear envelope and the cell's chromosomes have duplicated but are in the form of chromatin.
• In animal cells, two pair of centrioles formed from the replication of one pair are located outside of the nucleus.

Muscle Tissue

Heart Muscle Cell

Muscle tissue is made of "excitable" cells that are capable of contraction. Of all of the different tissue types, muscle tissue is the most abundant in most animals.

Muscle Tissue Types

Muscle tissue contains numerous microfilaments composed of actin and myosin, which are contractile proteins.

There are three major types of muscle tissue:

• Cardiac Muscle
  
  Cardiac muscle is so named because it is found in the heart. Cells are joined to one another by intercalated discs which allow the synchronization of the heart beat. Cardiac muscle is branched, striated muscle.
  
  
• Skeletal Muscle
  
  Skeletal muscle, which is attached to bones by tendons, is associated with the body's voluntary movements. Skeletal muscle is striated muscle. Unlike cardiac muscle, the cells are not branched.
  
  
• Visceral (Smooth) Muscle
  
  Visceral muscle, is found in various parts of the body such as the arteries, the bladder, the digestive tract, as well as in many other organs.
  
  Visceral muscle is also called smooth muscle because it doesn't have cross striations. Visceral muscle contracts slower than skeletal muscle, but the contraction can be sustained over a longer period of time.

Interesting Tidbits About Muscle Tissue

Interestingly, adults have a certain number of muscle cells. Through exercise, such as weight lifting, the cells enlarge but the overall number of cells does not increase.

Skeletal muscles are voluntary muscles because we have control over their contraction. Visceral muscles are involuntary since, for the most part, they are not consciously controlled.

Epithelial Tissue

Epithelial Cell

What are Tissues?

The word tissue is derived from a Latin word meaning to "weave." Cells that make up tissues are sometimes "woven" together with extracellular fibers.

Likewise, a tissue can sometimes be held together by a sticky substance that coats its cells.

There are four main categories of tissues: epithelial, connective, muscle and nervous. Let's take a look at epithelial tissue.

Epithelial Tissue

Epithelial tissue covers the outside of the body and lines organs and cavities. The cells in this type of tissue are very closely packed together and joined with little space between them.

With a tightly packed structure we would expect epithelial tissue to perhaps serve some type of barrier and protective function and that is certainly the case.

Epithelial tissue helps to protect organisms from microorganisms, injury, and fluid loss.

In an epithelium, the free surface is usually exposed to fluid or the air while the bottom surface is attached to a basement membrane.

Classifying

Epithelia are commonly classified based on the shape of the cells on the free surface, as well as the number of cell layers. Sample types include:

Simple Epithelium: A simple epithelium has a single layer of cells.

Stratified Epithelium: A stratified epithelium has multiple layers of cells.

Likewise, the shape of the cells on the free surface can be:

Cuboidal
Analogous to the shape of dice.

Columnar
Analogous to the shape of bricks on an end.

Squamous 

 Analogous to the shape of flat tiles on a floor.

By combining the terms for shape and layers, we can derive epithelial types such as stratified squamous epithelium or simple columnar epithelium.

Nervous Tissue

Nerve Cell

Nervous tissue is responsible for sensing stimuli and transmitting signals to and from different parts of an animal.

Nervous Tissue: Neurons

Neurons are the basic unit of nervous tissue..

A neuron consists of two major parts:

• Cell Body
  
  The central cell body contains the neuron's nucleus, associated cytoplasm, and other organelles.
  
  
• Nerve Processes
  
  Nerve processes are "finger-like" projections from the cell body that are able to conduct and transmit signals. There are two types:
  
  Axons - typically carry signals away from the cell body.
  
  Dendrites - typically carry signals toward the cell body.
  

Neurons usually have one axon (can be branched however). Axons usually terminate at a synapse through which the signal is sent to the next cell, most often through a dendrite.

Unlike axons, dendrites are usually more numerous, shorter and more branched. As with other structures in organisms, there are exceptions.

Bundles of axons and dendrites are called nerves. They are sensory if they consist of dendrites only, motor if they consist of axons only and mixed if they consist of both.

Connective Tissue

Connective Tissue

Connective Tissue

As the name implies, connective tissue serves a "connecting" function. It supports and binds other tissues. Unlike epithelial tissue, connective tissue typically has cells scattered throughout an extracellular matrix.

Loose Connective Tissue

In vertebrates, the most common type of connective tissue is loose connective tissue. It holds organs in place and attaches epithelial tissue to other underlying tissues.

Loose connective tissue is named based on the "weave" and type of its constituent fibers. There are three main types:

• Collagenous Fibers
  
  Collagenous fibers are made of collagen and consist of bundles of fibrils that are coils of collagen molecules.
  
• Elastic Fibers
  
  Elastic fibers are made of elastin and are stretchable.
  
• Reticular Fibers
  
  Reticular fibers join connective tissues to other tissues.

Fibrous Connective Tissue

Another type of connective tissue is fibrous connective tissue which is found in tendons and ligaments. Fibrous connective tissue is composed of large amounts of closely packed collagenous fibers.

Specialized Connective Tissues

Adipose

Adipose tissue is a form of loose connective tissue that stores fat.

Cartilage

Cartilage is a form of fibrous connective tissue that is composed of closely packed collagenous fibers in a rubbery gelatinous substance called chondrin. The skeletons of sharks and human embryos are composed of cartilage. Cartilage also provides flexible support for certain structures in adult humans including the nose, trachea and ears.

Bone

Bone is a type of mineralized connective tissue that contains collagen and calcium phosphate, a mineral crystal. Calcium phosphate gives bone its firmness.

Blood

Interestingly enough, blood is considered to be a type of connective tissue. Even though it has a different function in comparison to other connective tissues it does have an extracellular matrix. The matrix is the plasma and erythrocytes, leukocytes and platelets are suspended in the plasma.

The Cranial Nerves

Cranial Nerves

The cranial nerves are 12 pairs of nerves that can be seen on the ventral (bottom) surface of the brain. Some of these nerves bring information from the sense organs to the brain; other cranial nerves control muscles; other cranial nerves are connected to glands or internal organs such as the heart and lungs.

Cranial Nerves
Number	Name	Function	Location
I	Olfactory Nerve	Smell
II	Optic Nerve	Vision
III	Oculomotor Nerve	Eye movement; pupil constriction
IV	Trochlear Nerve	Eye movement
V	Trigeminal Nerve	Somatosensory information (touch, pain) from the face and head; muscles for chewing.
VI	Abducens Nerve	Eye movement
VII	Facial Nerve	Taste (anterior 2/3 of tongue); somatosensory information from ear; controls muscles used in facial expression.
VIII	Vestibulocochlear Nerve	Hearing; balance
IX	Glossopharyngeal Nerve	Taste (posterior 1/3 of tongue); Somatosensory information from tongue, tonsil, pharynx; controls some muscles used in swallowing.
X	Vagus Nerve	Sensory, motor and autonomic functions of viscera (glands, digestion, heart rate)
XI	Spinal Accessory Nerve	Controls muscles used in head movement.
XII	Hypoglossal Nerve	Controls muscles of tongue
Note: the olfactory "nerve" is composed of the rootlets of olfactory hair cells in the nasal mucosa and is not visible on the ventral surface of the brain. The rootlets end in the olfactory bulb. The olfactory tract contains nerve fibers projecting out of the olfactory bulb to the brain. The images in this table have been adapted from those in the Slice of Life project.

Hear IT!	Olfactory	Optic	Oculomotor	Trochlear
	Trigeminal	Abducens	Facial	Vestibulocochlear
	Glossopharyngeal	Vagus	Spinal Accessory	Hypoglossal

Can't remember the names of the cranial nerves? Here is a handy-dandy mnemonic for you: On Old Olympus Towering Top A Famous Vocal German Viewed Some Hops.
The bold letters stand for:
olfactory, optic, oculomotor, trochlear, trigeminal, abducens, facial, vestibulocochlear, glossopharyngeal, vagus, spinal accessory, hypoglossal.
Still can't remember the cranial nerves? Perhaps you need some Cranial Nerve Bookmarks to help you study! After you print the bookmarks, cut them into three individual bookmarks and use them to mark your place when you study.