DNA-Part3

Generation of new cells.

Chromosomes.

Cell Cycle.

Replication of DNA.

Cell division.

Interphase.

Mitosis.

Cytokinesis.

Reproductive cells.

Meiosis.

Fertilisation.

Sex determination.

Generation of new cells.

Cells have a finite life time.

New cells have to be continuously generated, to replace old cells.

We now know, how basic cell metabolism, is achieved with genes.

How does the body, create a new cell ?

Basically, it grows an existing cell, and divides it into two,

to create new cells.

New cells are always created, by cell division.

Can we cut a cell, into two to create new cells?

This might work for many organelles, in the cytoplasm.

Cell division, is not so simple for DNA.

We know that each cell, has to have exactly the same DNA.

We cannot cut DNA into half, and put it, into two divided cells.

If we do this, some genes will be present, and some absent, in each cell.

Without the proper genes, the cell would become dysfunctional.

Nature has evolved, a sophisticated mechanism,

to ensure that each divided cell has exactly the same DNA.

For this purpose genes, are organised into chromosomes.

Chromosomes play a critical role, in gene replication.

Chromosomes.

Human being have 46 chromosomes.

Except for reproductive cells, all other cells in the body,

has 46 chromosomes.

These cells which have 46 chromosomes, are called somatic cells.

A chromosome, has large number of genes.

It also has, a lot of fillers in between the genes.

23 of these chromosomes, are contributed by the father.

They are paternal chromosomes.

23 of these chromosomes, are contributed by the mother.

They are maternal chromosomes.

Each of the 23 chromosomes are unique.

They have a unique set of genes.

Each gene has some specific functionality.

Each unique paternal chromosome, has an equivalent maternal chromosome.

A set of paternal chromosome, and a maternal chromosome,

is called as a homologue.

Homologous chromosomes, have the same set of genes.

Homologous chromosomes, have same sequence, of genes.

Instead of saying, that a human being, has 46 chromosomes,

we can also say, that they have 23 pairs of chromosomes.

Each of the 23 chromosomes, have been uniquely identified.

They are numbered as chromosome 1, 2, 3, 4 and so on, up to chromosome 23.

Earlier, chromosomes were identified by their length.

The longest chromosome, was called chromosome 1.

The next longest chromosome, was called chromosome 2.

The shortest was called, chromosome 22.

The shortest chromosome, has about 50 million nucleotides.

The longest chromosome, has about 250 million nucleotides.

Further research, refined the length of the chromosomes.

But, the numbering of the chromosomes, remains the same.

Somewhere in the middle of the chromosome,

there is a centromere.

This divides the chromosome, into a larger and shorter arm.

The longer chromosomes typically have more genes.

The shorter chromosomes have smaller number of genes.

22 chromosomes have a matchable chromosome, in both sexes.

The gene sequence, in the matchable chromosome are same.

They are called autosomes.

Chromosomes 1 to 22 are autosomes.

Different chromosomes have different number of genes.

Some chromosome have more than 3000 genes.

Some chromosome have less than 1000 genes.

Chromosome 23 is special.

It is the sex determining chromosome.

Males have a X chromosome, and a Y chromosome.

Females have two X chromosomes.

Cell cycle.

The cell cycle goes through some distinct stages.

At one stage, the cell is involved in the routine functionality,

of synthesising proteins.

At some stage, it begins to focus on dividing itself.

Managing the cell metabolism, and dividing itself,

are important functionalities of a cell.

Cell division is the basic mechanism, for generating new cells.

Cell division, is an important functionality, of a cell.

Cell division is required, when a fertilised egg, develops into a baby.

Cell division is required, when a baby grows into an adult.

Cells have a finite life time.

Even in adults, new cells are required, to replace expired cells.

This also requires cell division.

Cell division, is a critical function, of all living cells.

When a cell divides, it produces two daughter cells.

Once a cell divides, it can create, new organelles using DNA.

Dividing DNA is not so simple.

How do we divide, the 23 pairs of chromosomes?

Just imagine, if two copies of chromosome three,

lands up in one daughter cell,

and another daughter cell does not have chromosome three.

This will dramatically alter the functionality, of both daughter cells.

This kind of error will defeat the very basic purpose of cell division.

The cell will become dysfunctional.

The critical criteria for successful cell division,

is to have, exactly the same 23 pairs of chromosomes,

in each divided cell.

Nature has devised an unique, and elegant method to do this.

Once the DNA, has been successfully, and accurately replicated,

the daughter cells, have the know-how, to synthesise,

more organelles, of the cell, as required.

This results in building 2 new cells, which has the same functionality,

as the original single cell.

Replication of DNA.

Replication of DNA, is at the core of cell division.

DNA is the only molecule, in a cell, which can duplicate itself.

This is an unique property of DNA.

During DNA replication, the two strands of the double helix, gets separated.

Each separated strand has the exposed bases, A, T, C, and G.

The nucleus has free, deoxyribonucleotide triphosphate, molecules.

These molecules have, a base, a deoxyribose sugar, and a triphosphate.

The exposed bases, in the single strand DNA,

base-pair with the free, deoxyribonucleotide molecules.

They then link together, to form a new strand of DNA.

The end result is, 2 identical molecules of DNA.

Each of them is called a copy.

In each copy, one strand of nucleotides, was present, in the original DNA molecule,

and one strand has been, newly synthesised.

Prior to cell division, the DNA molecules, in the nucleus,

are replicated by this process.

During replication each chromosome replicates itself.

The human genome has 23 pairs, or 46 chromosomes.

23 chromosomes, are paternal chromosomes.

23 chromosomes, are maternal chromosomes.

After replication, the same cell will have 92 chromosomes.

46 chromosomes, will be paternal chromosomes.

46 chromosomes, will be maternal chromosomes.

One copy is passed on to each of the two daughter cells.

Each daughter cell will also have 46 chromosomes.

Thus, the daughter cells receive, the same set of instructions,

present in the parent cell.

It is important to note that in replication, there is no change,

in genetic information, between the original cell, and the daughter cells.

Cell division.

There are three important stages, in cell division.

Interphase.

Mitosis.

Cytokinesis.

Interphase.

The period between the end of one division, and the beginning of the next division,

is known as interphase.

During most of the cells life time, it is in the state of interphase.

An important event, takes place, sometime during the interphase.

DNA replicates, during this phase.

During most of the interphase,

DNA exists in 46 threads, known as chromatin.

Each chromatin, has a set of genes.

The DNA in each chromatin thread, replicates at a pre determined time.

After replication, there will be, two identical threads, called sister chromatids.

The sister chromatids are joined together, at a point known as the centromere.

Each assumes an ‘X’ like shape.

They will be either paternal pair of sister chromatids,

or maternal pair of sister chromatids.

It is important to note, that each sister chromatid, will be genetically identical.

Before replication, there were 23 pairs of chromosomes.

All the 46 chromosomes, are replicated.

After replication, there will be 46 pairs, of sister chromatids.

Each sister chromatid, has identical genetic information.

Mitosis.

Mitosis comprises of 3 sub phases.

Prophase.

Metaphase.

Anaphase.

Prophase.

Nature has evolved, a very innovative process, to ensure,

that each daughter cell, receives exactly one copy,

of the sister chromatid.

The cell begins to erect a scaffold, around the nucleus, with two poles.

The poles are situated at opposite ends, of the cell.

This is called as the spindle apparatus.

The structure is comprised of protein assemblies, called microtubules.

The centromeres of each chromosome, attach to microtubules,

emanating from each pole.

Each chromatid, has a unique microtubules, to attach it to a pole.

One sister chromatid, gets connected to one pole,

while the other sister chromatid, gets connected to the opposite pole.

The nuclear membrane, that surrounds the chromosomes,

start to break down.

Metaphase.

The next phase, is called metaphase.

All the “X” shaped chromosomes line up in the middle of the cell.

It is like the equator of the cell.

This plane at the centre, is called as the metaphase plate.

All the 46 pairs of sister chromotids, neatly line up along the metaphase plate.

One sister chromatid, faces one pole,

and other sister chromatid, faces the opposite pole.

The microtubule connects them, to the pole.

Anaphase.

The next phase, is called as the anaphase.

The sister chromatids, split at the connection point, the centromere.

The separated sister chromatids, move towards the opposite poles.

The cell is now ready, to split.

Cytokinesis.

The cell begins to constrict at a point, till the cell divides into half.

Two daughter cells are created.

Each daughter cell has 46 of the divided chromatids.

Dividing each of the 46 pairs, of sister chromatids,

also ensures one more critical factor.

Each daughter cell, has 23 pairs of homologous chromosomes.

Each pair has a paternal chromosome, and a maternal chromosome.

This is how it was, in the original cell.

So nature manages to achieve, an exact replica, of the original cell.

The spindle fibres dissolve.

A nuclear envelope forms.

This completes the process of cell replication.

We started with one cell.

We now have two daughter cells.

The daughter cells have identical genetic information.

Considering that this process happens quadrillion number of times,

it is amazing that nature is able to successfully, repeatedly, and accurately,

executes this process, of cell division.

The normal function of the human body, is heavily dependent,

on successful cell division.

Reproductive cells.

Reproduction of life, starts with a single fertilised egg.

Fertilisation occurs, when a male reproductive cell,

fuses with a female reproductive cell.

Reproductive cells, are called as gametes.

The male reproductive gamete cells, are called as spermatozoa.

A single male reproductive cell, is called as a sperm in short.

The female reproductive gamete cells, are called as ova.

A single female reproductive cell, is called as ovum.

There is a fundamental difference,

between cell replication,

and production of reproductive cells.

In cell replication, the genetic information,

in the daughter cells, are identical.

When generating reproductive cells, nature shuffles,

the maternal and paternal chromosomes.

The population of cells, that give rise to gametes, or reproductive cells,

are known as germ cells.

Each germ cell, contains 23 paternal chromosomes,

and 23 maternal chromosomes.

Gametes are produced from germ cells.

There are some important differences, between gametes and other cells.

Gametes have only 23 chromosomes.

They will have all the chromosomes,

from chromosome 1, to chromosome 23.

But they will have only one copy, of each chromosome.

Sperm cells will have only 23 chromosomes.

Ovum cells will have only 23 chromosomes.

A normal human cell has 23 pairs of chromosomes.

For this to happen, sperm cell has to fertilise an ovum cell.

There is another important difference, between normal cells, and gametes.

Each gamete cell, is a random mix of paternal and maternal chromosomes.

The father shuffles ‘his’ father’s and mother’s chromosomes.

Each sperm cell, is a different combination of paternal and maternal chromosomes.

The mother shuffles ‘her’ father’s and mother’s chromosomes.

Each ovum cell, is a different combination of paternal and maternal chromosomes.

It is important to note, that this shuffling takes place,

even before the sperm meets an ovum.

If and when, a sperm cell fertilises, a ovum cell,

the resulting cell, is called as a zygote.

The zygote has chromosomes, inherited from,

the father’s mother and father,

and the mother’s mother and father.

The main process involved in producing gametes, is meiosis.

Meiosis consists of two cell divisions, in succession.

The cell carries out, one round of DNA replication.

It then carries out, two rounds of cell division.

When it started out, the germ cell had 46 chromosomes.

After DNA replication, it has 92 sister chromatids.

After the first meiotic division, each of the two cells,

has 46 sister chromatids.

After the second, meiotic division, each of the four cells,

has 23 chromosomes.

Meiosis.

The process of meiosis, has multiple steps.

Step 1:

During the interphase, the forty six chromosomes are copied,

resulting in 46 pairs of X shaped chromosomes.

Each pair has two sister chromatids.

Step 2:

Each replicated chromosome finds its homologue.

The paternal chromosome finds its homologues maternal chromosome.

The homologues lock together.

Now, there are 23 pairs of matched, locked homologues.

These are called bivalents.

Each bivalent has one pair of paternal sister chromatids,

and one pair of maternal sister chromatids.

DNA gets exchanged between the replicated chromosomes.

Step 3:

The nuclear envelope breaks down.

The matched, locked together chromosome complex,

or bivalent, moves to the metaphase plate.

The spindle apparatus attaches the centromere of one replicated chromosome,

to one pole of the cell.

It attaches the centromere of the other replicated chromosome,

to the other pole of the cell.

Step 4:

The two chromosomes, that make up each bivalent,

get pulled to the opposite poles of the spindle.

Let us call these poles, as pole A and pole B.

The maternal and paternal sister chromatids,

will be randomly oriented,

to the poles A and B.

The orientation will determine, to which cell,

the sister chromatids, finally end up in.

Sister chromatids, in the replicated chromosome stay together.

Step 5:

Cytokinesis divides the cell, into two cells.

Now, we have two cells, from one cell.

Each cell has 23 pairs of sister chromatids.

Because of the random orientation in step 4,

it is extremely unlikely that all the paternal chromosomes,

will land up in one cell.

It is equally unlikely all the maternal chromosomes,

will land up in one cell.

Each cell will receive a random combination,

of paternal and maternal chromosomes.

We note, that this results, in a shuffling of maternal and paternal chromosomes.

Each gamete cell, has a different combination,

of maternal and paternal chromosome.

This cell division, is called as the first meiotic cell division.

Step 6:

In each cell, replicated chromosome move to the metaphase plate.

The spindle apparatus, now attaches to the centromeres,

of both sister chromatids.

These are attached to the opposite poles of the spindle.

Step 7:

The two sister chromatids separate,

and move to the opposite ends of the cell.

Step 8:

The nuclear membrane reforms,

and cytokinesis separates the cells, into two separate cells.

Now, we have four cells, from one cell.

Each cell has 23 single unreplicated chromosomes.

Summarising, Meiosis produces daughter cells with only 23 chromosomes.

Genetic variation, of gametes.

Nature seems to have, a specially designed, different process,

to produce gamete cells.

The essence of these seems to be genetic variability,

in each gamete cell.

Each gamete cell has 23 chromosomes.

Each chromosome can be paternal or maternal.

We can calculate the number of possible variants.

It can result in 2 to the power of 23,

which is about 8 million different combinations or variants.

It is apparent, that nature goes out of the way, to create variants,

even before it reproduces.

It makes it almost impossible, to produce a clone of either parent.

Fertilisation.

The unfertilised ovum is released, into the uterine tube.

The ovum takes about 4 days, to reach the uterus.

If a sperm cell manages to meet, with the ovum, during this period,

fertilisation can happen.

If fertilisation, has to take place, it has to be in the uterine tube.

In contrast with the ovum, which is released one at a time,

during intercourse, hundreds of millions of sperm cells are generated.

Only a few hundred of them, can manage to reach the uterine tube.

Fertilisation, is the fusion of a sperm and a ovum.

23 chromosomes from the ovum, join 23 chromosomes from the sperm.

They eventually become a single cell.

This fertilised ovum, is called as a zygote.

The zygote contains half the DNA from one parent,

and another half from the other parent.

The child therefore, inherits part of the genes, of each parent.

There is a large amount, of gene shuffling that takes place,

in each parent germ cell, before a zygote is formed.

Due to this, the resulting zygote, is one of the several million combinations,

of the DNA, of the parent cells.

Even when the same parents, have multiple children,

each child will be a different and unique combination, of parent’s DNA.

This accounts for the differences of children, we notice, of the same parents.

We carry the genes, not only from our parents,

but also from both pairs of our grandparents.

We carry some of the genes, from our ancient forefathers.

We even carry some genes, from early forms of life, like bacteria.

The zygote reaches the uterus, and starts developing into a baby.

It is called, as an embryo in the first two months.

After that, it is called as the foetus.

In about 38 weeks, a baby is born.

A new life is created.

Sex determination.

There are 23 pairs of chromosomes, in the DNA.

One of these pairs, is responsible for sex determination.

These are called as X and Y chromosomes.

Males possess, one X and one Y chromosome.

Females have two X chromosomes.

The sex of an individual, is determined at the time of fertilisation.

The ovum cell, which has two X chromosomes,

always contributes a X chromosome.

The sperm can contribute, either a X or a Y chromosome.

If the sperm contributes a X, the resulting zygote, will be XX,

which will be a female.

If the sperm contributes a Y, the resulting zygote, will be XY,

which will be a male.

There is equal probability of a zygote, being male or female.

This is how nature ensures, that about half the population,

will be male or female.