Structure of DNA
-DNA is a double-stranded polymer of nucleotides (= polynucleotide)
-A nucleotide is made of pentose sugar called deoxyribose, a phopshate group and an organic nitrogenous base.
-There are two types of bases
1.Purines- adenine and guanine
2.pyrimidines-thymine and cytosine
-two nucleotides join together by a condensation reaction. The two polynucleotide chains are held together by hydrogen bonds between the bases.
How DNA is well suited to carry out its functions
- it's a very stable molecule and can pass from generation to generation without change.
-the two strands are able to separate because of hydrogen bonding and this enables the DNA to be replicated before the cell divison
-the base pairs are held within the sugar-phosphate backbone, so the genetic information is well protected
-Dna molecules are very large so they can carry a large amount of genetic information
the structure of RNA
-RNA is a single stranded polymer of nucleotide
-RNA contains the pentose sugar, ribose
-RNA contains the organic nitrogenous bases but thymine is replaced by uracil
1.mRNA is a long, single-stranded molecule formed into a helix. It is manufactured in the nucleus and carries the genetic code from the DNA to the ribosomes
2.tRNA is a small single-stranded molecule. tRNA molecules transport amino acids to the ribosome so that proteins can be synthesised
3.rRNA is found in the cytoplasm and is a large, complex molecule made up of both double and single helices. Ribosomes are made up of ribosomal RNA and protein.
-made of DNA, protein and a small amount of RNA
-shortly before cell division begins each DNA molecules makes a copy of itself and the single thread the DNA becomes two identical threads called the chromatids. They lie parallel along most of their length but they are joined in a specialised area called the centromere.
somatic cells have 23 pairs of chromosomes, so they are diploid
sex cells have 23 chromosomes and they are haploid
Mitosis is a type of cell division and it produces two genetically identical daughter cells that are both diploid.
Mitosis is used for asexual reproduction, growth and repair
-the longest part of the cell cycle
-during interphase, a cell increases in size, produces organelles lost during previous division. Additionally, DNA is duplicated and a synthesis of ATP/protein happens again
This is the longest stage in mitosis.
-the chromosomes condense and become visible. They are now referred to as pairs of chromatids.
-the nuclear membrane disintegrates and the nucleolus disappears
-centrioles divide and move to the opposite poles of the cell
-protein microtubules form from each centriole and the spindle develops, extending from pole to pole
During metaphase, the chromosomes arrange themselves at the centre or equator of the spindle and become attached to some spindle fibres at the centromere
Anaphase is a very short stage,
During anaphase, the spindles contract and chromatids are pulled apart
Telophase is the final stage of mitosis.
Chromosomes, now located at different ends of the cell, uncoil and become unvisible again.
The spindle breaks down, centrioles replicate, the nucleoli reappears and the nuclear membrane reforms.
In an animal cell, the cytokinesis occurs by the constriction of the centre of the parent cells from the outside inwards.
In plant cells, a cell plate forms across the equator of the parent cell from the centre outwards and a new cell wall is laid down.