Summer break holiday

homework

Now that we have learned how living things are organized what’s next?

. We will learn more about the lowest level of organization: cells

What is the first thing that we need to know about cells?

All cells fall into one of the two major classifications of either prokaryotes or

eukaryotes.

Prokaryotes

• Prokaryotes are organisms made up of cells that lack a cell nucleus or any membrane-encased organelles. This means the genetic material DNA in prokaryotes is not bound within a nucleus. Additionally, the DNA is less structured in prokaryotes than in eukaryotes. In prokaryotes, DNA is a single loop. In Eukaryotes, DNA is organized into chromosomes. Most prokaryotes are made up of just a single cell (unicellular) but there are a few that are made of collections of cells (multicellular). Scientists have divided the prokaryotes into two groups, the Bacteria and the Archae.

What do prokaryotic cells look like?

• Prokaryotes are the simplest type of cell. (appeared about four billion years ago) .

• Prokaryotes are the largest group of organisms Prokaryotes unicellular organisms that are found in all environments.

• Prokaryotes do not have a nuclear membrane . Their genetic material dispersed throughout cytoplasm. Prokaryotes do not have membrane-bound organelles Prokaryotes have a simple internal structure.

Shapes of Prokaryotes

• Eukaryotes are organisms made up of cells that possess a membrane-bound nucleus (that holds genetic material) as well as membrane-bound organelles. Genetic material in eukaryotes is contained within a nucleus within the cell and DNA is organized into chromosomes. Eukaryotic organisms may be multicellular or single-celled organisms. All animals are eukaryotes. Other eukaryotes include plants, fungi, and protists.

What do eukaryotic cells look like?

What Prokaryotic and Eukaryotic Cells Have in Common

• Both have DNA as their genetic material (it’s DNA that tells cells what kind of cells they should be).

• Both are covered by a cell membrane,.

• Both are made from the same basic chemicals: carbohydrates, proteins, nucleic acid, minerals, fats and vitamins.

• Both have ribosomes , Both contain RNA.

• Both have similar basic metabolism (life processes) like photosynthesis and reproduction.

• Both are highly regulated by elaborate sensing systems ("chemical noses”) that make them aware of the reactions within them and the environment around them.

DIFFERENCES BETWEEN PROKARYOTIC AND EUKARYOTIC CELLS

parametersEukaryotic Cell Prokaryotic Cell

Nucleus: Present Absent

Number of chromosomes: More than oneOne--but not true chromosome: Plasmids

Cell Type: Usually multicellularUsually unicellular (some cyanobacteria may be multicellular)

True Membrane bound Nucleus:

Present Absent

Example: Animals and Plants Bacteria and Archaea

Genetic Recombination: Meiosis and fusion of gametesPartial, undirectional transfers DNA

Lysosomes and peroxisomes:

Present Absent

Microtubules: Present Absent or rare

Endoplasmic reticulum: Present Absent

Mitochondria: Present Absent

Cytoskeleton: Present May be absent

DNA wrapping on proteins.:

Eukaryotes wrap their DNA around proteins called histones.

Multiple proteins act together to fold and condense prokaryotic DNA. Folded DNA is then organized into a variety of conformations that are supercoiled and wound around tetramers of the HU protein.

Ribosomes: larger smaller

Vesicles: Present Present

Golgi apparatus: Present Absent

PARAMETERS EUKARYOTIC CELLS PROKARYOTIC CELL

Chloroplasts: Present (in plants)Absent; chlorophyll scattered in the cytoplasm

Flagella:

Microscopic in size; membrane bound; usually arranged as nine doublets surrounding two singlets

Submicroscopic in size, composed of only one fiber

Permeability of Nuclear Membrane:

Selective not present

Plasma membrane with steriod:

Yes Usually no

Cell wall:Only in plant cells and fungi (chemically simpler)

Usually chemically complexed

Vacuoles: Present Present

Cell size: 10-100um 1-10um

PARAMETERS EUKARYOTIC CELLS PROKARYOTIC CELLS

•NOW LET US LEARN MORE ABOUT CHROMOSOMES AND CHROMATIN

CHROMOSOMES AND CHROMATIN

CHROMOSOMES

• A chromosome is an organized structure of DNA and protein that is found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions. The word ''chromosome'' comes from the Greek (''chroma'', color) and (''soma'', body) due to their property of being very strongly stained by particular dyes.

Human chromosomes

• Chromosomes in humans can be divided into two types: autosomesand sex chromosomes. Certain genetic traits are linked to a person's sex and are passed on through the sex chromosomes. The autosomescontain the rest of the genetic hereditary information. All act in the same way during cell division. Human cells have 23 pairs of chromosomes (22 pairs of autosomes and one pair of sex chromosomes), giving a total of 46 per cell

• Packed inside the nucleus of every human cell is nearly 6 feet of DNA, which is subdivided into 46 individual molecules, one for each chromosome and each about 1.5 inches long. Collecting all this material into a microscopic cell nucleus is an extraordinary feat of packaging. For DNA to function when necessary, it can't be haphazardly crammed into the nucleus or simply wound up like a ball of string. Consequently, during interphase, DNA is combined with proteins and organized into a precise, compact structure, a dense string-like fiber called chromatin, which condenses even further into chromosomes during cell division.

• Each DNA strand wraps around groups of small protein molecules called histones, forming a series of bead-like structures, called nucleosomes, connected by the DNA strand (as illustrated in Figure 1). Under the microscope, uncondensed chromatin has a "beads on a string" appearance. The string of nucleosomes, already compacted by a factor of six, is then coiled into an even denser structure known as a solenoid that compacts the DNA by a factor of 40. The solenoid structure then coils to form a hollow tube. This complex compression and structuring of DNA serves several functions. The overall negative charge of the DNA is neutralized by the positive charge of the histone molecules, the DNA takes up much less space, and inactive DNA can be folded into inaccessible locations until it is needed.

TYPES OF CHROMATIN

• There are two basic. Euchromatin is the genetically active type of chromatin involved in transcribing RNA to produce proteins used in cell function and growth. The predominant type of chromatin found in cells during interphase, euchromatin is more diffuse than the other kind of chromatin, which is termed heterochromatin. The additional compression of heterochromatin is thought to involve various proteins in addition to the histones, and the DNA it contains is thought to be genetically inactive. Heterochromatin tends to be most concentrated along chromosomes at certain regions of the structures, such as the centromeres and telomeres. Genes typically located in euchromatin can be experimentally silenced (not expressed) by relocating them to a heterochromatin position.

CHROMATIN VS CHROMOSOME

• In the nucleus, the DNA double helix is packaged by special proteins (histones) to form a complex called chromatin. The chromatin undergoes further condensation to form the chromosome. So while the chromatin is a lower order of DNA organization, chromosomes are the higher order of DNA organization. An organism’s genetic content is counted in terms of the chromosome pairs present. e.g. humans have 23 pairs of chromosomes.

parameters Chromatin Chromosome

Definition:

In the nucleus, the DNA double helix is packaged by special proteins (histones) to form a complex called chromatin. The chromatin undergoes further condensation to form the chromosome.

A threadlike structure of nucleic acids and protein found in the nucleus of most living cells, carrying genetic information in the form of genes.

Structure:

Composed of nucleosomes-a complex of DNA and proteins (called histones). Represent DNA folded on nucleoproteins by a magnitude of 50. The chromatin fiber is app. 10 nm in diameter.

Chromosomes are condensed Chromatin Fibers. They are a higher order of DNA organization, where DNA is condensed at least by 10,000 times onto itself.

Appearance:

Chromatin Fibers are Long and thin. They are coiled structures found inside the nucleus.

Chromosomes are compact, thick and ribbon-like. These are uncoiled structures found inside the nucleus

DIFFERENCES BETWEEN CHROMOSOMES AND CHROMATIN

PARAMETERS CHROMATIN CHROMOSOMES

Pairs: Chromatin is unpaired. Chromosome is paired.

Metabolic activity:

Permissive to DNA replication, RNA synthesis (transcription) and recombination events.

Refractory to these processes

Presence:Found throughout the cell cycle.

Distinctly visible at metaphase as highly condensed structures in nucleus upto several thousand nm.

Visualization:Electron microscope (beads on string appearance)

Light microscope

Conformation:

May have open (euchromatin) or compact (heterochromatin) conformations, which is dynamically regulated during the cell-cycle stages.

Predominantly heterochromatic state with a predetermined position in the nucleus and a specific shape such as metacentric, acrocentric, telocentric.

• Prepared by : GEETU

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