DNA structure, function and replication
Function of DNA
Long term storage of information
Life processes
Movement
Nutrition
Growth
Reproduction
Responding to stimuli
Excretion
Respiration
Contains the instructions to construct other components of the cells such as proteins and RNA molecules
DNA segments that carry genetic information are called genes.
Structure of DNA
Double helix consisting of two strands coiled like spring
nucleotides
repeating units
consists of sugar molecules, a phosphate group and a nitrogenous base
chains of nucleotides form the sugar-phosphate backbone
4 bases
adenine
thymine
guanine
cytosine
adenine always forms weak hydrogen bonds with thymine on the adjacent DNA strand of the double helix
Guanine always forms weak hydrogen bonds with cytosine
hydrogen bonds are weak so they can break and reform easily when the DNA replicates
Complementary base pairing is important
the structure of DNA remains exact and orderly. Hydrogen bonding between the two polynucleotide strand ensures that the DNA molecule does not form an irregular structure that would prevent its packaging and biological function in the cell
The DNA molecule can store large quantities of genetic information as the molecule is very large. Base pairing enables separation of the polynucleotide strands, so the information stored can be used in replication and transcription
RNA
single stranded
contains ribose sugar
contains uracil
shorter
DNA in Eukaryotic cells
wound tightly around globular proteins called histones, forming DNA- proteins mixture called chromatids
Chromatids are organised into linear structures called chromosomes
Chromosomes
two or more in the cell nucleus
karyogram- taken when chromosomes are condensed and visible. The chromosomes in the photo can be cut out, arranged in order of size, numbered and pasted to produce a composite photo.
Genes are located in specific positions on a chromosome. These positions are called gene's locus or gene loci
DNA in prokaryotic cells
single, circular chromosome located in a region of the cytoplasm called the nucleoid
Prokaryotes are single celled organisms
Prokaryotic genome is smaller than eukaryotic genome
Prokaryotes do not contain histone but do have proteins that assist in the spatial organisation of the chromosome in the nucleoid
DNA replication
DNA in parent cell is replicated before cell division occurs to ensure that ensure that each daughter cell receives an exact copy of the genetic information
in the nucleus of eukaryotic cells and the cytoplasm of prokaryotes
The sequence of bases on DNA makes up a code of instructions called the genetic code. The code provides the information needed to instruct the cell to carry out its functions. This code can be passed on to new cells
DNA replication is described as a semi-conservative process as the new DNA molecule each consist of one original strand and one newly synthesised strand.
Steps
- The complementary strands of the double helix are unwound and separated to expose the bases
- A enzyme named helicase breaks the weak hydrogen bonds between the original DNA molecule
- Free DNA nucleotides are added to the exposed bases on the exposed strands, Hydrogen bonds are formed between the free nucleotides and the bases on the exposed strands.
- DNA polymerase joins the sugar and phosphate groups of adjacent nucleotides creating the sugar-phosphate backbone of the newly synthesised polynucleotide strand
- DNA polymerase checks that the new sequence is complementary to the original sequence and removes and replaced misplaced nucleotides