#studentsnotes

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Humans have 23 pairs of chromosomes, for a total of 46 chromosomes.

1-Length

nanometre (nm)

micrometre (μm)

millimetre (mm)

centimetre (cm)

metre (m)

kilometre (km)

2-Mass

gram (g)

kilogram (kg)

tonne (t)

3-Area

square metre (m²)

square centimetre (cm²)

hectare (ha)

4-Volume

cubic centimetre (cm³)

cubic metre (m³)

millilitre (mL)

litre (L)

5-Time

second (s)

6-Temperature

degree Celsius (°C)

7-Force

newton (N)

8-Energy

joule (J)

kilojoule (kJ)

9-Pressure

pascal (Pa)

kilopascal (kPa)

10-Electric current

ampere (A)

11-Quantity of electric charge

coulomb (C)

12-Frequency

hertz (Hz)

13-Power

Ribosomes are made in the nucleolus and are found both free in the cytoplasm and attached to the membranes that form the RER. Ribosomes are usually present in large numbers.

The size of ribosomes is often recorded in Svedberg units (symbol, S). This is a measure of their rate of sedimentation during centrifugation under standardised conditions, rather than their actual size.

Ribosomes in the cytoplasm of eukaryotic cells have a sedimentation rate of 80S (an actual size of about 25 nm diameter). Those of prokaryotic cells (and of those found within both mitochondria and chloroplasts) are slightly smaller, with a sedimentation rate of 70S.

Ribosomes are built of two sub-units, and do not have membranes as part of their structures. Chemically, they consist of protein and the nucleic acid RNA.Each ribosome is composed of two oddly shaped subunits—a large one and a small one.

Common Features:

1. Function

Ribosomes are the sites at which protein synthesis occurs.

2. Components

Protein and rRNA are the building blocks for ribosomes. Each of the forms of rRNA and most of the proteins are present in only one copy per ribosome.

3. Subunits

Ribosomes have two subunits, one large and one small.

Differences:

Eukaryotic ribosomes are larger than prokaryotic ribosomes. In prokaryotes, the 70S ribosome is made up of a large subunit (5OS) and a small subunit (30S), whereas eukaryotes have an 8OS ribosome containing a large subunit (60S) and a small subunit (40S).

Eukaryotic ribosomes contain four forms of rRNA, whereas prokaryotes have only three forms. Other minor differences are found in the number of proteins found in both the large and small subunits of eukaryotic and prokaryotic systems.

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The nucleus contains most of the genes in the eukaryotic cell. (Some genes are located in mitochondria and chloroplasts.) It is generally the most conspicuous organelle, averaging about 5 μm in diameter. The nuclear envelope encloses the nucleus, separating its contents from the cytoplasm. The nuclear envelope is a double membrane. The two membranes, each a lipid bilayer with associated proteins, are separated by a space of 20–40 nm.

The envelope is perforated by pore structures that are about 100 nm in diameter. At the lip of each pore, the inner and outer membranes of the nuclear envelope are continuous. An intricate protein structure called a pore complex lines each pore and plays an important role in the cell by regulating the entry and exit of proteins and RNAs, as well as large complexes of macromolecules.

Except at the pores, the nuclear side of the envelope is lined by the nuclear lamina, a netlike array of protein filaments that maintains the shape of the nucleus by mechanically supporting the nuclear envelope. There is also much evidence for a nuclear matrix, a framework of protein fibers extending throughout the nuclear interior. The nuclear lamina and matrix may help organize the genetic material so it functions efficiently.

Within the nucleus, the DNA is organized into discrete units called chromosomes, structures that carry the genetic information. Each chromosome contains one long DNA molecule associated with many proteins. Some of the proteins help coil the DNA molecule of each chromosome, reducing its length and allowing it to fit into the nucleus.

The complex of DNA and proteins making up chromosomes is called chromatin. Chromatin inside a eukaryotic cell nucleus can be classified into two major types based on its state of condensation: heterochromatin and euchromatin.

A prominent structure within the nondividing nucleus is the nucleolus (plural, nucleoli), which appears through the electron microscope as a mass of densely stained granules and fibers adjoining part of the chromatin.

The nucleus is involved in both mitosis and meiosis. Several events required for cell division occur in the nucleus, including DNA replication and transcription of DNA into precursor RNA molecules.

All of the enzymes required for replication and repair of newly synthesized DNA, as well as for transcription and processing of precursor RNA molecules, are found in the nucleus.