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

  1. The complementary strands of the double helix are unwound and separated to expose the bases
  1. A enzyme named helicase breaks the weak hydrogen bonds between the original DNA molecule
  1. 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.
  1. DNA polymerase joins the sugar and phosphate groups of adjacent nucleotides creating the sugar-phosphate backbone of the newly synthesised polynucleotide strand
  1. DNA polymerase checks that the new sequence is complementary to the original sequence and removes and replaced misplaced nucleotides