Topic 3 : Genetics
3.2 Chromosomes
Prokaryotes:
- Prokaryotes have circular DNA without association of protein.
- There is one copy of each gene except when the cell and its DNA are replicating
- Plasmids are small separate (usually circular) DNA molecules located in some prokaryotic cells
- Plasmids are also naked (not associated with proteins) and are not needed for daily life processes in the cell.
- The genes in plasmids are often associated with antibiotic resistant and can be transferred from one bacterial cell to another.
- Plasmids are readily used by scientists to artificially transfer genes from one species to another (ie. Gene for human insulin)
Plasmid features:
- Naked DNA without association of protein such as histone
- Small circular ring of DNA
- Not responsible for normal life process
- Contain survival characteristics, e.g. antibiotic resistence
- Can be passed on between bacteria
- Can be incorporated into nucleoid chromosomes (save permanently)
Eukaryotes:
- Eukaryotic chromosomes are linear and are made up of DNA and histone proteins.
- Histones are globular shaped protein in which the DNA is wrapped around.
- Linear chromosomes vary in length, centromere location and genes containing
- In humans there are 23 types of chromosomes. There are 22 pairs of autosomes. The 23rd pair are the sex chromosomes. Males have an X and a Y chromosome and females have two X chromosomes
- Each chromosome carries a specific sequence of genes along the linear DNA molecule. The position where the gene is located is called the locus
- The number of chromosomes is known as N number.
- Normal cell contains diploid nucleus – 2N(two pairs of homologous chromosomes)
- Sex cell contains haploid nucleus – N(one pairs of homologous chromosomes)
- The chromosome number is an important characteristics of the specie
Homologous chromosome:
- Homologous chromosomes are chromosomes within each cell that carry the same genes at the same loci
- One chromosome came from an individual’s mother and one from the father
- They have the same structure and size
- These chromosomes pair up during meiosis
- Even though these chromosomes carry the same genes, they could have different alleles
Sex chromosome:
- The X and Y chromosome determine the sex of an individual
- The X chromosome is quite large in comparison to the Y chromosome
and has a centromere that is located near the centre or middle of the chromosome - The Y chromosome is relatively small with its centromere located near the end of the chromosome
- If an individual has two X chromosomes they will be a female and if they have an X and a Y chromosome they will be a male
- All other chromosomes are called autosomes and do not affect the sex of an individual
- SRY genes on Y chromosomes lead to male development
- Using a karyogram, we distinguish sex, it shows the chromosomes of an organism in homologous pairs of decreasing length.
