DNA structure

What is DNA

  • The generic material that determines the characteristics of a species and is passed onto offspring for the continuity of life

 

Basic DNA Structure:

 

  • Composed of two helical stranding winding around a central axis

  • Individual subunits that make the strands are nucleotides, which have a sugar phosphate backbone and a base facing the centre of the double helix

 

DNA organization:

DNA molecules are very long, wrapping around proteins called histones which organizes them to fit into the microscopic anucleus in eukaryotic cells

 

Coiled DNA stored as chromatin (in non-dividing cells) which are further coiled into chromosomes(in dividing cells)

 

Coiling DNA around histones:

  • First, DNA wraps around proteins called histones grouped together in 8's

  • DNA loops around histones twice before looping around the next histone

  • 8 histones and DNA wrapped around it is called a nucleosome

  • Creates thick cord called chromatin

  • Chromatin then coils further to create chromosomes

 

Types of chromatin:

Looking at a chromosome, it often appears striped, this banding is because chromatin can be differentiated into two regions:

 

Heterochromatin:

  • Very tightly coiled and dense, absorbing more microscopic staining and looking darker

Euchromatin:

  • Less tightly coiled appearing lighter

 

Chromosome Structure:

 

 

Telomere:

  • Is the length of DNA at the ends of a chromosome that prevent damage to the chromosome during cell division

  • As cells divide the telomeres continue to get shorter

  • As they shorten more damage accumulates in the chromosome, causing aging

Centromere:

  • Chromosomes have a constriction, where they appear thinner

  • Is the place where  chromatids join to form a double chromosome

Loci:

  • A genetic location is called a locus

  • A locus is the position of a gene, gene cluster or even a single nucleotide on a chromosome

  • Since there are two of each chromosome in a somatic cell, there are two copies of each gene

  • Individual gene copies are called alleles, given names such as D and d depending on Gene characteristics

 

 

Double vs single chromosome:

A double chromosome is made up of two chromatids, once the cell divides and these are pulled apart they are called (single) chromosomes

 

Types of chromosomes:

Autosomes or somatic chromosomes are the same in both males and females

 

Autosomes are in pairs, these pairs are called homologues chromosomes. Normally one is inherited from the mother and one from the father

 

Heterosomes are non identical chromosomes that pair up at meiosis. These are typically sex chromosomes which affect sexual traits

 

Human chromosomes:

Ploidy refers to the number of pairs in a cell

  • Human somatic(body) cells contain 46 chromosomes. This is called the diploid number (2n) of chromosomes.

  • Human gametes (sex cells) contain 23 chromosomes, called the haploid number (n) of chromosomes

In humans, the first 22 pairs of chromosomes are autosomes, the 23rd pair can be matched or unmatched (X and Y). These are heterosomes/ sex chromosomes.

 

Location of chromosomes: Eukaryotes

  • Human chromosomes exist as single lengths this is the same for all eukaryotes

  • DNA is found in the nucleus of eukaryotes

  • It is also found within mitochondria and chloroplasts however is exists here as circular rings

Location of chromosomes: Prokaryotes

  • Have unbound single large haploid circular chromosome in the cytosol

  • Bacteria also contain smaller sections of DNA free floating in cytoplasm known as plasmids, often carry non essential genes that may benefit survival of the organism.