Organisation and Genomic Structure

Haploid

Haploid organisms only have one set of chromosomes, therefore their characteristic number is n.

Diploid

Diploid organisms have two sets of chromosomes, their characteristic number is 2n.

Karyotype

A spread of chromosomes at mitotic metaphase, chromosome 1 is largest.

Metacentric

Centromere is in the middle

Acrocentric

Centromere is near the end

Telocentric

Centromere is very near the end, humans do not have these.

Nucleolar Organiser

Appears as constrictions similar to a centromere, is called a secondary constriction. It is located in different positions in different species and it contains a cluster of genes that code for rRNA.

Nucleoli

Organelles in the nucleus containing rRNA and ribosome components, resides next to the nucleolar organiser.

Chromatin

A DNA and protein complex, it is not uniformly distributed along the whole chromosome length.

Euchromatin

Loosely packed chromatin, rich in genes, high recombination frequency

Heterochromatin

Tightly packed chromatin, few genes, low recombination frequencies.

Chromatin structure

There are five histone proteins, one is external to the core and the rest are within the core.

Chromosome banding patterns - Giesma

These are produced from chromosome staining - giesma. There is G-light and G-dark, they relate to chromatin density. Heterochromatin stains dark.

Eukaryote genome

Vary 5000-fold from largest to smallest, there is a huge variation in genome size compared to gene number with most eukaryotes having 20000 genes.

Why do mammals have low gene density?

The remaining genome which does not code for protein is used for higher complexity of organisms.

What is the human genome made up of?

1.5% exons, 20% introns, 5% regulatory sequences, 15% unique noncoding DNA.

More than 50% is repetitive sequence.

Unique Sequence DNA of the human genome

Made up of introns, exons, regulatory sequences, and non coding DNA.

Non coding DNA sits between genes and is often genes that are no longer functional (pseudogenes)

Telomeric repeats

This is part of the repetitive DNA of the human genome. Telomeres are caps at the end of chromosomes, these degrade with each cell division. Telomeres are comprised of many tandem arrays of a simple DNA sequence (TTAGGG)

Satellite repeats

This is part of the repetitive DNA of the human genome. These are highly repeated tandem sequences which are usually heterochromatic, short AT-rich tandem repeats. These include microsatellites (repeat unit 2-6bp) and minisatellites (15-100bp repeats)

Dispersed gene families

This is part of the repetitive DNA of the human genome. Dispersed meaning the genes are scattered across the genome. E.g. the globin gene family - a-globin spread across chromosome 16 and b-globin spread across chromosome 11.

Tandem gene families

This is part of the repetitive DNA of the human genome. Some cells need large amounts of product of some genes, therefore genes have arranged tandem, often times they have identical DNA sequences.

Gene family

Genes that are similar but not identical, they have related function and arise through duplication.

Visualisation of telomeres

This is done by using a probe with a fluorescent label that binds via sequence complementarity. The sequence of DNA of the probe will bind to relative telomeric DNA sequence, allowing them to be visualised.

Mitosis

In somatic cells. Two daughter cells produced with same chromosome number as parent (2n). Homologues dont interact, there is no crossover.

Meiosis

In gametes. Four daughter cells produced with half the DNA (n). There is crossing over in M1 and independent assortment to create genetic variation.

Linkage

This is when genes are close together on the same chromosome and alleles are inherited together.

Linkage terminology - how are alleles on the same chromosome set out

With no punctuation between them.

AB or ab

Linkage terminology - how are two homologues set out

With a dash between them

A/a

Linkage terminology - what order are alleles written in

The same order on each homologue

Linkage terminology - unlinked genes are set out

With a semi colon between then

A/a ; C/c

Linkage terminology - genes of unknown linkage

Separated by a dot

A/a . D/d

How to know if genes are linked from a testcross?

If the ratio is not 1:1:1:1 and is instead >50% parental class and <50% recombination class.

What are cis and trans allele configuration?

Cis - two dominant alleles on the same homologue.

Trans - one dominant and one recessive allele on the same homologue