DNA

Almost all the DNA of eukaryotic cells is found in the linear chromosomes of the nucleus. Small amounts of circular DNA are found in cytoplasmic organelles such as mitochondria & chloroplast.

The bases in DNA have carbon-nitrogen ring structures (nitrogenous bases)

Adenine & Guanine (double ring nitrogenous bases) are purines.

Thymine and Cytosine (single ring nitrogenous bases) are pyramidines

• Adenine (A)-6-amino purine,
• Cytosine (C)-2 -oxy-4-aminopyrimidine,
• Guanine (G)-2-amino-6-oxopurine,
• Thymine (T)-2, 4-dioxy-5-methylpyrimidine

Nucleoside vs Nucleotide

• Sugar + base=Nucleoside (The combination of a base with the sugar constitutes a nucleoside)
• Sugar + base (Nucleoside) + phosphate=Nucleotide

• In DNA, the sugar is a form of ribose called deoxyribose (it is missing the oxygen in the 2’ carbon atom)

Chargaff’s rule 

He found that in DNA the concentration of adenine always equalled the concentration of thymine and the concentration of guanine always equalled the concentration of cytosine ie. the amount of purine=the amount of pyramidine in a given DNA molecule.

A/T or G/C=1, A=T, G=C

A+T=C+G (No. of AT pairs need not equal to the number of GC pairs) 

Bonds in DNA : Hydrogen Bonds and Phosphodiester Bonds

• Adenine is paired to Thymine by 2 hydrogen bonds 
• Cytosine is paired to Guanine by 3 hydrogen bonds 

 DNA contains many mono deoxyribo nucleotides covalently linked by 3’5’-phosphodiester bonds. The resulting long, unbranched chain has polarity, with both a 5’end and a 3’end that are not attached to other nucleotides.

CROSSING OVER

Crossing over is the exchange of segments between the non-sister chromatids of homologous chromosome. Don’t confuse this term with translocation. Translocation is the movement of segments between non-homologous chromosomes or region of the same chromosome. An example may fix the issue. Philadelphia chromosome; a chromosome formed due to reciprocal translocation between chromosome 9 and 22. A portion of chromosome 9 translocated to chromosome 22 that is between two different chromosomes. This is translocation. O.K.  Fine. 

Types of crossing over

The term crossing over was coined by Morgan. Crossing over can even occur in somatic cells. Somatic crossing over is rare and has no significance. Somatic crossing over is reported in Drosophila by Curt Stern (1935). Now moving into the detail of meiotic crossing over or germinal crossing over; this type of crossing over takes place in germinal cells during gametogenesis. This process is universal in occurrence and has great significance as we discussed in the introductory paragraph.

Mechanism of meiotic crossing over

Crossing over is a crucial process that generates genetic difference within a population.  The prerequisites for crossing over are firstly, 99.7% of DNA replication and 75% of histone synthesis must occur by prophase I. Secondly, each chromosome must attach by its telomeres (ends of the chromosome) to the nuclear envelope through specialized structures called attachment plaques. 

The major steps in meiotic crossing over are 

1) synapsis 

2) duplication of chromosome 

3) crossing over and 

4) terminalisation.

                Synapsis is the intimate pairing between the two homologous chromosomes. It is initiated during the zygotene stages of prophase I of meiosis I. Here chromosomes are aligned side by side and each gene has its counterpart aligned perfectly (gene for gene alignment). The resultant pairs of homologous chromosomes are called bi-valents.

Synapsis is followed by duplication of chromosome (in pachytene). Sister chromatids are held at centromere. At this stage, each bi-valent has four chromatids now called as tetrad.

Crossing over or exchange of segments between the non-sister chromatids of homologous chromosome occurs at the tetrad stage. Homologous chromosome may stay in synapsis for even days during pachytene stage. Now let us have a look into the details of crossing over. 

For easier understanding, crossing over can be divided into three major steps:

1) breakage of chromatid segments 

2) their transposition (movement to the respective site) and 

3) fusion or joining.

Which are the enzymes involved in crossing over?

1) Recombinase is the major enzyme regulating recombination event

2) Endonuclease 

3) Ligase enzyme
Endonuclease is responsible for breakage of 2 non-sister chromatids at corresponding sites. This is followed by the exchange of segments and finally the exchanged segments are joined or the gap is filled by ligase enzyme. 

Crossing over takes place at several points on a tetrad and result in several chiasmata. These are regions were chromosomes are held together.  Larger the chromosome size the more the number of chiasmata. Frequency of crossing over is dependent on the physical distance between genes on the chromosome. The chance of crossing over is more for distantly located genes.

                The final step is terminalisation. After crossing over the non-sister chromatids starts to repel each other. During diplotene, Synaptonemal complexdissolves and desynapsis takes place. During diakinesis, chromosome detaches from the nuclear envelope and the chromatids separates progressively from the centromere towards the chiasmata. Meanwhile chiasma itself moves in a zipper fashion towards the end of tetrad. This movement of chiasma is known as terminalisation. As a result of terminalisation, homologous chromosomes are separated completely.

REPRODUCTION IN ORGANISMS

Diversity in living Organisms

1. The production o

f new organisms from the

existing organisms ofthe same species is

known

as reproduction.

2. Asexual Reproduction :

Modes of sexual

reproduction used are binary fission,

multiple

fission, Budding, spore formation, regeneration,

vegetative propagation, tissu

e

culture,

fragmentation

3. Sexual Reproduction :

In sexual reproduction, a

male gamete (germ cells) fuses with a

female

gamete to form a new cell called ‘zygote’. This

zygote then grows and develop into a

new

organism in due course of time.

When male game

te and female gamete fuse,

they form a zygote and the process is known

as

fertilization.

Fertilization is of two types :

-

External

fertilization and Internal fertilization.

4. Sexual Reproduction in Flowering Plants :

Flower is meant essentially for sexual

reproduction.



Pollination is the process in which pollen

grains are transferred from the anther to

stigma

of the carpel. It is of two types self

pollination and cross

-

pollination.



In the fertilization process primary

endospermic nucleus is formed.



After the fertilization process, ovary

developes into the fruit whereas ovules into

the

seed.

5. Reproduction in Human Being :

The sex organ

in males are testes and ova in females.



Male reproductive organ consist of a pair

of testes, vasdeferens, a pair of

epididymis,

a

pair of ejaculatory duct, urethra, pairs of

accessory gland.



Female reproductive part consist of a pair

of ovaries, a pair of fallopian tube,

uterus,

vagina, external genitalia, mammary glands

and accessory glands. Ovary

produces the female

gametes (eggs or ova) and female sex hormone (estrogen).



If sperms are present, fertilization of ovum

takes place in the upper end of the

fallopian

tube.



Bleeding accompanied by discharge of soft

tissue lining the reproductive tract

is

menstrual flow. It l

ast for 3

-

5 days.



Secretory phase lasts for 12

-

14 days.



Fertilization process occurs in fallopian

tube. In this process zygote is formed. In

this

process umbilical cord is produced

which is attached to foetus. During this



process two hormones are produced

which

are estrogen and progesterone.

Progesterone stops mensturation and

prevents ovulation. The placenta protects

the body

against diseases. Due to

contraction of uterine muscles young one

is expelled and the

phenomenon is called

parturition.



If the egg i

s not fertilized, it lives for about

one day. Since the ovary releases one

egg

every month, the uterus also prepares it self

every month to recieve a fertilized egg.



After the age of 45

-

50 years menses stop

and process is called menopause.



Fertility contro

l can be done chemically,

mechanically or surgically.

6. Reproductive Health

Barrier methods

(i) Mechanical barrier method:

-

They prevent

contraception by preventing either sperms

from

entering uterus or preventing

implantation if fertilization has occurre

d.

The instruments are condom, cercival cap,

diaphragm & ICDU method.

(ii) Hormonal method: They are used by

women for suppressing the production of

ovum. i.e. ,

oral pills, Implants morning after

pills.

(iii) Chemical contraception: They are

creams, jelli

es and foaming tables which

are placed in

vagina for killing the sperms

at the time of coitus.

(iv) Surgical techniques:

—

(a) Vasectomy:

—

The two vasa deferential

of the male are blocked by cutting a small

piece of

tying the rest. This prevents the

passag

e of sperms from testes to semen.

(b) Tubectomy:

—

A portion of both the

fallopian tubes is excised to ligated to block

the

passage of ovum

7. Sexually Transmitted Diseases (STDs)

It is a group of infections caused by different

types of pathogens that are t

ransmitted by

sexual

contact between a healthy person and an

infected person. The sexually transmitted

diseases are

also called venereal diseases (VDs).

Some 30 different types of STDs are known. Fol

example :

Gonorrhoea, Syphilis, Trichomonas,

Genital war

ts, AIDS.