1.1 DNA structure

DNA

Deoxyribonucleic acid

Macromolecule found in all living cells

two stranded

double helix

Chromosomes and DNA

DNA in cells is found in chromosomes

DNA stores and transmits genetic information

Functions the same way in all living things

DNA from one species can be inserted into the genome from another ad its message can still be transmitted

Prokaryotic cells

DNA is located within the nucleoid region of the cytosol

circular

one chromosome

double stranded

plasmids are small circular DNA that is seperate from chromosomal DNA

Eukaryotic Cells

DNA is located in the nucleus

somatic cells have 46 chromosomes in 23 pairs

22 pairs of autosomal (non-sex) chromosomes and one pair of sex chromosomes

Chromosomes are arranged into condensed linear strands consisting of double stranded DNA molecule and histone proteins

Decondensed form of chromosomes is called chromatin

Genetic code must be accessible so that DNA synthesis, protein synthesis and RNA synthesis can occur

If DNA molecule is condensed the genetic code is inaccessible sp the processes cant occur.

DNA structure

Made up of repeating called nucleotides

• 5 carbon sugar (deoxyribose)

• Phosphate group

• 1 of 4 bases: adenine (A), cytosine (C), thymine (T) and guanine (G)

Phosphate of one nucleotide is attached to the sugar of another, making the sugar phosphate backbone

One strand ends with phosphate - the 5’ prime end- and the other ends with the sugar - the 3’ end.

Each base forms a weak hydrogen bond with its complementary base

Weak hydrogen bonds are important because DNA replication involves breaking apart the double helix.

A bonds to T

G bonds to C

DNA Replication

Occurs in both prokaryotic and eukaryotic cells

Nucleoid region in prokaryotes

Nucleus in eukaryotes

DNA replication is semi-conservative

each double strand consists of one old and one new strand

Helicase breaks the hydrogen bonds and unwinds DNA

DNA polymerase adds nucleotides in the 5’ to 3/ direction to the old strands

The product is 2 identical DNA strands containing the exact same DNA as the original strand.

After DNA replication, DNA is doubled and all chromosomes in the cell consist of 2 identical sister chromatids

DNA replication occurs prior to cell division so that each daughter cell has identical DNA to the parent cell.

Amount of DNA in original cell doubles during cell replication but number of chromosomes stays the same.