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Base pair

2013-10-21T19:52:47Z

120049529:

A base pair is made up of 2 complemetary bases, joined by Hydrogen bonds. Complementary bases include: A-T and C-G. There are 2 hydrogen bonds between A and T and 3 hydrogen bonds between C and G (can be remembered as there are 3 letters between C and G in the alphabet).

Base pair

2013-10-21T19:52:13Z

120049529: Created page with "A base pair is made up of 2 complemetary bases, joined by Hydrogen bonds. Complementary bases include: A-T and C-G.There are 2 hydrogen bonds between A and T and 3 hydrogen bonds..."

A base pair is made up of 2 complemetary bases, joined by Hydrogen bonds. Complementary bases include: A-T and C-G.There are 2 hydrogen bonds between A and T and 3 hydrogen bonds between C and G (can be remembered as there are 3 letters between C and G in the alphabet). 

Chiral carbon

2012-11-29T12:26:16Z

120049529: Created page with " A chiral carbon is a carbon atom which has 4 different groups attached, this makes the molecule assymetric and produces two (or more) isomers."

 A chiral carbon is a carbon atom which has 4 different groups attached, this makes the molecule assymetric and produces two (or more) isomers.

Protein structure

2012-11-29T12:24:00Z

120049529:

[[Proteins|Proteins]] are made up of polymers of [[Amino acids|amino acids]]. The amino acids are joined together by [[Peptide bonds|peptide bonds]] in a [[Condensation reaction|condensation reaction]]. This series of peptide bonds is also known as the [[Polypeptide|polypeptide]] backbone, off which are side chains made up of amino acids. This type of reaction is catalysed by the [[Ribosome|ribosome]] in the [[Cytoplasm|cytoplasm]] and releases a [[Water|water]] molecule. There are four levels of protein structure. The [[Primary structure|primary structure]] is the linear sequence of [[Amino acids|amino acids]] joined by covalent peptide bonds. <ref>http://www.chemguide.co.uk/organicprops/aminoacids/proteinstruct.html</ref>. The main [[Secondary structure|secondary structures]] of proteins are the [[Alpha-helix|alpha helix]] and the [[Beta pleated sheet|beta pleated sheet]]. The secondary protein sturcture is stabilised by hydrogen bonds which are 1/10 the strength of covalent bonds. The protein then continues to fold forming a three-dimensional structure. This is known as the [[Tertiary structure|tertiary structure]]. Disulphide bridges are formed in the tertiary structure, these are formed between amino acids which contain a thiol group (SH). The H+ is lost very easily in an oxidation reaction with another SH group and the two join with a disulphide bond. Proteins that have to work outside of the cell use disulphide bonds to increase their stability. If two or more tertiary structures form a single structure then it is a [[Quaternary structure|quaternary structure]]. An example of a quaternary structure are [[Haemoglobin|haemoglobin]] molecules, that are made up of four [[Globin|globin]] [[Molecules|molecules]]. These are also known as [[Red blood cells|red blood cells]] and are found in [[Blood|blood]] <ref>Alberts et al., (2008) Molecular Biology of the Cell, 5th Edition, Garland Science, Chapter 3, Page 136</ref>.

Proteins can come in all different shapes and sizes <ref>Alberts et al.(2008) Molecular Biology of the Cell, 5th Edition, Garland Science Chapter 3 Page 144</ref> due to the fact that there is any possible sequence of [[Amino acid|amino acids]] and that a protein can be made of an alpha helix, a beta pleated sheet or both. The amino acids which tend to be conserved in proteins are those which make up the [[Enzyme active site|active site]], as this is the part of the protein which has most functional significace. 

=== References ===

<references />

Meiosis

2012-11-23T14:59:15Z

120049529:

Meiosis is a type of cell division. It can only occur in [[Diploid|diploid]] cells, resulting in four unidentical [[Haploid|haploid]] [[Daughter cells|daughter cells]].This contrasts to mitosis which can occur in both [[Haploid|haploid]] and [[Diploid|diploid]] cells, producing only two identical daughter cells. Therefore meiosis results in variation where as mitosis produces exact copies of the parent cell. There are two stages of Meiosis, Meiosis I and II. This leads to the nucleus dividing twice but chromosome replication only occuring once. Like mitosis, chromosomes in meiosis have duplicated in Interphase, during S phase.  

= Meiosis I <ref>Hartl DL and Jones EW (2009) Genetics: Analysis of Genes and Genomes, Seventh Edition, USA, Jones and Bartlett Publishers</ref> =

Meiosis 1 is separated into 4 stages. These are Prophase I, Metaphase I, Anaphase I and Telophase I.

== Prophase I ==

During prophase I chromosomes pair, condense and crossing over occurs between non-sister chromotids. It is separated into 5 different stages. Similar to mitosis, cetrioles move to opposite poles and spindle fibres start to form. 

=== Leptotene ===

Leptotene is the first stage of Prophase I. During the Leptotene stage chromosomes coil and condense. This is whne the chromosomes first become visible.

=== Zygotene ===

During Zygotene the homologous chromosomes pair up and the [[Synaptonemal Complex|synaptonemal complex]] between the homologous chromosomes start forming.

=== Pachytene ===

Synapsis is now complete and the a [[Bivalent|bivalent]] (pair of homologous chromosomes) is formed. Chiasmata (singular: chiasma) form between non-sister chromotids of homolohous chromosomes. This is the point where crossing over occurs and DNA exchange occurs. 

=== Diplotene ===

The snaptonemal complex breaks down, allowing the chromosomes to separate. The chiasmata are now visible and are point at which the chromosomes are still held together. 

=== Diakensis ===

During diakenesis the nuclear envelope starts to breakdown. The bivalents are now ready for metaphase 

== Metaphase I ==

Bivalents or tetrads (four chromotids) align on on the metaphase plate (the equator of the cell) and spindle fibres attach to the kinetichores, protein structures located at the centromeres. 

== Anaphase I ==

Disjunction occurs. This is when the mitotic spindles pull the tetrads apart to forming dyads, which migrate to opposite poles. 

== Telophase I ==

Nuclear envelope may form around the dyads and cytokinesis (cell division) occurs. 

== Interphase ==

Interphase does not have to occur between Telophase I and Prophase II, but it can occur. Unlike traditional interphase, there is no DNA replication but growth and biosynthetic activities can still occur. 

= Meiosis II   =

= Prophase II =

Unlike Prophase I no chiasmata form and no crossing over occurs. If a nuclear envelope has formed during Telophase I, it is broken down. Centroiles move to opposing poles and spindle fibres start to form. 

== Metaphase II ==

The dyads align on the metaphase plate and spindle fibres attach to the kinetechores.

== Anaphase II ==

The dyads are pulled apart by spindle fibres and the myads arrive at opposite poles.

== Telophase II ==

The nuclear envelope reforms around the myads. Cytokenesis occurs and the cells divide. This leaves us with 4 unidentical daughter cells, also known as gametes. 

= References =

<references />

Paramecium Cells

2012-11-23T14:34:15Z

120049529:

[[Eukaryotic cells|''Paramecium'' cells]] are unicellular organisms. They are part of the [[Eukaryotes|eukaryotic]] family. Thus they have membrane-bound [[Organelles|organelles]].

They can also be categorised as Ciliates, since they are surrounded by numerous [[Cilia|cilia]]; these assist the cytostome for the nutrition process of ''paramecium ''and also the movement of the cell. They also have anal pores for excretion processes and [[Contractile vacuole|contractile vacuoles]] whose function prevents bursting of the cell, which would otherwise be caused by the [[Osmosis|osmotic]] uptake of water, by a process known as [[Osmoregulation|osmoregulation]] <ref name="(Beale G. & Preer J,2008).">Beale, Goffey and Preer, John R. Jr. (2008) Paramecium Genetics and Epigenetics CRC Press, Taylor and Francis Group.</ref>. The Paramecium cell reproduces by a process called [[Reproduction by Conjugation|conjugation]].<ref>L.Prescott, J.Hardley and D.Klein Microbiology 6th Edition New York:McGraw-Hill</ref>

''Paramecium'' cells are very large. The length of a typical [[Cell|''paramecium'']] varies from 100 μm to 300 μm <ref>Brock Biology of Micro-organisms 12th Edition, Madigan Dunlap Clark, Pg 69</ref>. They can be found in freshwater areas, like ponds and lakes. 

=== References: ===

<references />