Packaging of DNA

Packaging DNA

Histones: alkaline (basic) proteins that wrap DNA

Nucleosomes: DNA wrapped twice around a core of 8 histones

30-300nm fibre: fibres formed from nucleosomes

Chromatin: the densely packed DNA

Chromosome: visible only during mitosis/meiosis

In a human somatic (diploid) cell, the total length of DNA contained in the nucleus is about 2 metres when fully stretched out. This is because each cell contains roughly 6 billion pairs of DNA.

Homologous Chromosomes

Humans have 46 chromosomes which are composed of 23 pairs of homologous chromosomes, one coming from each parent,

Homologous chromosomes pair up, cross over and separate during meiosis to form gametes

During fertilization homologous chromosomes combine again to form the new zygotes’ genome

Homologous chromosomes have the same genes at the same locations, but as they come from a different parent, the alleles can be different.

Genetic Recombination

Many genes will be found on a single chromosome

There can be crossing over between different alleles as a result of meiosis

Genetic recombination involves alignment of two homologous chromosomes and equal exchange of DNA segments between the two strands

The closer the genes are, the less likely recombination is going to be (frequency of recombination is lower)

Cross over values (COV) are used to express how often crossing over occurs (and thus also tell us how far away two genes are on a chromosome)

Frequency of recombination: 15%= not too far apart

Frequency of recombination: 10%= close enough together

Frequency of recombination: 50%= genes on diff. chromosomes

Xtra notes

Nucleic Acids and Adenine are present in genes, nucleosomes, and chromatin

At the zygote stage, there is only one genome.

A human has the same genome in the liver and kidney cell, and almost all other cells as well. The ER and ribosome don’t gave a genome.

Genomes made up of RNA are found exclusively in viruses.

Dominance

Incomplete Dominance: occurs when neither allele is completely dominant, so the heterozygote shows an intermediate (blended) phenotype. No dominant/ recessive allele

Codominance: both alleles are fully expressed in the heterozygote, traits appear side by side, no dominant or recessive allele.