Chromosome Nomenclature, Types & Structure - PowerPoint Presentation

Slide1

Chromosome Nomenclature, Types & Structure

Slide2

Chromosomes

The

term chromosome was coined by W.

Waldeyer

in 1888

.

Their

structures

varies in

different organisms

like

they are circular in

bacteria

but

linear in eukaryotic organisms, however there number remains constant for a species.

Slide3

Size of

Chromosomes

The

size

varies

from species to species

or

even within the same

species.

The

size

of chromosome is normally

measured at

mitotic metaphase

stage where they obtained their maximum

volume.

In

most of the organisms, their size range from

0.1 to 30

μm

in length

and

0.2 to 2.0

μm

in thickness

.

Slide4

Organisms with

less number of chromosomes contain relatively

larger sized

chromosomes.

In general, chromosomes

in case of plants

are

larger in size

than in animals.

Among plants, cells of monocots contain larger chromosomes than cells of

dicot

plants.

Slide5

Shape of chromosomes

Shape

is

changeable from phase to phase during cell growth and cell division

i.e. the

interphase

chromosomes occur in the form of

thin, coiled

, elastic, contractile thread like structure (a kind of chromatin threads).

However

,

during metaphase and anaphase

they become

thick and filamentous

.

Slide6

A chromosome has two arms which are held together at a point called

centromere

.

The position of this centromere varies that provides different shapes.

If the centromere is present at the centre giving two equal arms then the chromosome will assume

'V' shape

.

But if the centromere is slightly away from the centre with one arm is shorter than the other arm assumes an

'L' shape

or

'J' shape

, however, when the centromere is present on the terminal point having only one arm and other arm is absent, they assumes a

'I' shape

.

Slide7

Types of chromosomes

:

Based

on the

position of the

centromeres

chromosomes are of

four

types

:

1.

Telocentric

2.

Acrocentric

3

.

Sub-

metacentric

4.

Metacentric

Slide8

1.

Telocentric

where

centromere

present on the proximal end. They look like

rod shaped

 

2.

Acrocentric

where

centromere

present near one end, thus one arm is very short while the other arm is

ver

long. They also appear as rod shaped

3

.

Sub-

metacentric

where

centromere

occur near the centre resulting in two unequal arms, so they appear as L or J shaped)

4.

Metacentric

chromosomes where

centromere

occur in centre thereby forming two equal arms becoming V shaped.

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Based on functions, they are two types:

(1)

Autosomes

and

(2)

Sex chromosomes.

Slide12

(1)

Autosomes

:

F

ound in all the body cells

and they

decide the somatic characters

of the organism.

In humans, there are 46 chromosomes out of which 44 are

autosomes

and the remaining two are sex chromosomes.

Slide13

(2) Sex chromosomes:

Involved in the

sex determination

.

In both

animals

and plants, these chromosomes are found different in both structure and functions.

In human males there are two sex chromosomes as XY and in human female there are two sex chromosomes as XX.

Slide14

Unusual of Chromosomes

:

Beside these normal chromosomes, certain animals and plants possess one or more chromosomes which are quite different from the normal chromosomes in their basic structure, size and shape.

These

chromosomes are called unusual of chromosomes, examples

B-chromosomes

.

The

other name of B-chromosomes is accessory and supernumerary chromosomes.

These

chromosomes are actually extra chromosomes that are found in certain individuals especially in plants like in case of maize.

Slide15

Special types of chromosomes

:

Found

in certain specialized eukaryotic cells.

They

are very large in size and hence called as giant chromosomes.

They

are of two types

(

1)

Polytene

chromosomes

and

(

2) Lamp brush

chromosomes

Slide16

(1)

Polytene

chromosomes

First discovered by

C.G.Balbiani

in the year 1881 in the

salivary glands of drosophila

.

Hence called as

salivary gland chromosomes

.

They have an

extremely large puff in the center

called as Chromosomal puff or

Balbiani

ring.

Along their body, they have

alternat

light bands

(clear zones) and

dark bands

respectively like in a zebra.

Slide17

(2) Lamp brush chromosomes

First discovered by

Flemming

in the year 1882.

They got this name as they resemble the

shape of a brush

.

They are generally seen in the

diplotene

stage

of the

meiotic prophase

in the

oocytes

of

Salamandor

and in the giant nuclei of the unicellular algae

Acetabularia

.

They have an extremely condensed central axis from which lateral loops made up of DNA extend outwards.

This is due to the intense RNA synthesis.

Slide18

Number of

chromosomes

Each species of plants and animals has a definite constant

somatic

and

gametic

number of chromosomes

.

The

somatic

or body cells contain

2 diploid set

of chromosomes which is represented as

2n

.

This

diploidy

is achieved by the union of two haploid male and female gametes

in sexual reproduction

.

Slide19

The

Gametic

chromosome number

is the number of chromosomes found in the

gametes

of a species.

It is precisely

one half of the somatic number

and is

represented by n

commonly called as

haploid number

.

So somatic chromosome number is commonly known as the

diploid number.

The total number of genes present in a haploid set of chromosomes is called genome

.

Therefore one haploid set has one genome, whereas, the diploid cell will have two genomes.

Slide20

Structure of Chromosomes:

A typical chromosome has similar structures called

chromatids

.

Since

they are similar, they are called sister

chromatids

.

The

chromatids

are connected to the center part called

centromere

.

A

centromere

is a spherical structure which is a network is protein

fibres

.

Slide21

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Slide23

The

centromeres

vary in number in different chromosomes

.

A chromosome with only

one centromere is called

monocentric

and that one

with many

centromeres

is called polycentric

.

Slide24

The structure of chromosome varies during various phases of cell cycle, thus, various term are used to define the structure of chromosomes:

1.

Chromonemata

:

During mitotic

prophase

, when the chromosomal material becomes visible as very thin filaments, they are called as

chromonemata

.

Chromonemata

is actually an earlier stage of

chromatid

that consists of a single linear DNA molecule with associated proteins. It is gene bearing portion.

Slide25

2.

Chromatids

:

At mitotic

metaphase

each chromosome consists of 2 symmetrical or similar structures called as

chromatids

.

Since they are similar, they are called sister

chromatids

.

Each

chromatid

contains a single DNA molecule.

Both

chromatids

are connected to the center part called

centromere

and become separated at beginning of anaphase when sister

chromatids

of a chromosome migrate to opposite pole.

Slide26

3.

Chromomere

:

These are regions of tightly folded DNA.

These are bead like accumulations of chromatin material visible along

interphase

chromosome.

Chromomere

become clearly visible in

polytene

chromosome where they become aligned side-by-side constituting chromosome

beeds

.

However, at metaphase, the chromosomes are tightly coiled therefore, these

chromomeres

become invisible.

Slide27

4.

Centromere

&

Kinetochore

:

There are of two types of constrictions, primary and secondary constrictions.

The

primary constriction is made up of

centromere

and

kinetochore

.

A

centromere

is a spherical structure contain a network is protein

fibres

.

Centromeres

vary in number in different chromosomes i.e. chromosome with only one

centromere

is called

monocentric

while with many is called polycentric

.

 

Slide28

Centromere

consists of specific DNA sequences with special proteins forming

kinetochores

.

So,

kinetochore

is a plate or cup like disc situated upon primary constriction or

centromere

.

Centromere

is the region to which are attached the

fibres

of mitotic spindle and allows the

chromatids

to move.

So during mitosis, 4-40 microtubules of mitotic spindle become attached to

kinetochore

and provide the force for chromosomal movement.

If the

centromere

is damaged, then the chromosome cannot move in the anaphase stage.

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Slide33

5.Telomeres:

Each extremity of chromosomes has a polarity and therefore it prevents other chromosomal segments to be fused with it.

The chromosomal ends are known as Telomeres. If a chromosome breaks, the broken ends can fuse with other chromosome due to lack of the Telomeres.

Slide34

6. Secondary Constriction:

In addition to primary constriction or centromere the chromosomes may have some more constrictions at any point on the chromosomes.

All such constrictions other than the primary constriction are called secondary constrictions.

Slide35

7.

Nucleolar

organizers:

There are certain secondary constrictions that contain genes coding for 5.8 s, 18 s and 28 s ribosomal RNA that induce formation of nucleoli.

Such

secondary constriction that gives rise to the nucleoli formation are called as

nucleolar

organizers.

In

human beings

nucleolar

organizers are located in secondary constrictions of chromosomes 13, 14, 15, 21, and 22 all of which are

acrocentric

and have satellites

.

Slide36

A small structure attached to the lateral end of the chromosome by a secondary constriction is called

satellite

and such chromosomes with the satellite are called SAT-chromosomes.

Centromere

is another lateral structure attached to the chromosome.

It gives stability to the chromosome.

Slide37

Slide38

A typical chromosome has the satellite at its one end and the

centromere at the other end.

A chromosome is made up of

DNA

, RNA,

histone

and non-

histone

protons

and some of them also have metallic ions like Ca2+ and Mg2+.