DNA, Epigenetics and Cell Division

DNA

The molecule that contains the genetic information responsible for the development and function of an organism

Histone

A protein that provides structural support to the chromosome

Nucleosome

DNA wrapped around 8 histones

Chromatin

Coiled DNA + histones (many nucleosomes)

Chromosome

Tightly coiled chromatin that can be seen with a light microscope during cell division

Gene

A section of DNA that codes for a particular protein

Structure of DNA

Histones are proteins that help DNA to condense. DNA wraps around 8 histones to form a nucleosome. Nucleosomes fold in on themselves allowing DNA to coil into chromatin. In cell division, coiled chromatin becomes even more tightly coiled (super coiled) hence forming chromosomes.

How nucleotides form a single chain

The nucleotides of one strand are joined together by a bond between the phosphate group of one nucleotide and the deoxyribose sugar of another. This forms the phosphate sugar backbone.

How nucleotides form a double chain

Nitrogenous bases are attracted to their complimentary base pair hence forming a second strand. Hydrogen bonding between the two strands keeps them together.

Similarities of Nuclear DNA and Mitochondrial DNA

• Both are made of DNA, composed of the same 4 nucleotides

• Both contain genes that encode for proteins essential for cellular function

Nuclear DNA

• Located in the nucleus

• Shaped as long strands

• Bound to proteins

• Function is to code for proteins responisble for the functioning of the cell

Mitochondrial DNA

• Located in the mitochondria

• Circular in shape

• Not bound to proteins

• Function is to code for proteins reponsible for the functioning of the mitochondria.

Similarities of DNA and RNA

• Both form a phosphate-sugar backbone

• Both made of nucleotides

• Nitrogenous bases are the same except for the fact that U replaces the T in RNA

DNA in comparison to RNA

• Sugar molecule is deoxyribose sugar

• Double stranded

• Nitrogenous bases : A—T, G—C

• No folding

RNA is comparison to DNA

• Sugar molecule is ribose sugar

• Single stranded

• Nitrogenous bases : A—U, C—G

• Can fold in on itself forming H-bonds between complimentary base pairs

mRNA

• Stands for messanger RNA

• Located in the nucleus and cytoplasm

• Structure is complimentary to the DNA sequence

• Its function is the carry messages of genentic code from the nucleus to the ribosome

rRNA

• Stands for ribosomal RNA

• Location is in the ribosome

• Structure of rRNA makes up 60% of the mass of ribosomes

• Its function is to ensure the correct alignment of mRNA, tRNA and ribosome. It also has an enzymatic role in forming the peptide bond.

tRNA

• Stands for transfer RNA

• Located in the cytoplasm

• Structured as a small folded molecule (70-90 nucleotides) carryings specific nucleotides

• Its function is to carry specific amino acids

Structure of proteins

Proteins are amino acids held together by peptide bonds. The sequence of amino acids determines the type of protein being made.

Relationship between DNA and proteins

Types of proteins a cell can make is determined by genes. A sequence of three bases on DNA (triplet code) codes for a particular amino acid

Triplet

3 bases on the DNA strand

Codon

3 bases on the mRNA strand

Template strand

• DNA strand is being copied, that is, transcribed to made mRNA

• mRNA is complimentary to the template strand

Coding strand

• Complimentary DNA strand to the template strand

Helicase

Helicase unwinds and unzips DNA by breaking hydrogen bonds between complimentary base pairs. This forms a template strand and a coding strand

RNA polymerase

RNA polymerase adds complimentary nucleotides to the template strand composing complimentary mRNA strand. RNA uses uracil instead of thymine.

Translation

The process of building a protein at the ribosome using the code in the mRNA

Start codon

The first codon of mRNA translated by a ribosome (AUG)

End codon

A codon on mRNA that marks the end of translation (UAA, UAG, UGA)

Anticodon

Three bases on tRNA

Examples of proteins

• Haemoglobin

• Actin and myosin

• Fibrin

• Collagen

• Insulin

• Amylase

Epigenetics

Changes in gene expression that results from mechanisms other than change in gene (environmental factors)

Epigenome

The sum of all the facots that determine when, where and which genes will be expressed (switched on). Helps to control which genes are actives in a particular cell

Gene Expression

• Which genes are turned on - this will determine the cells functioning

• Certain genes will by switched on in certain cells

• All genes have the same genes but their function depends on whether the gene is turned on or not

Description of Methylation

• The addition of a methyl group to a histone or the DNA (CpG site)

• Tag can be added to DNA molecule

• At the CpG site = cytosine - phosphorus - guanine : That is where a cytosine nucleotide is adjacent to a guanine nucleotide

• Tag can be added to histone tail

• DNA coil tighter around histones

• DNA polymerase access restricted and hence inhibits transcription