The Genome & Chromosomal Structure

The Gene and the Genome

• Prokaryotic chromosomes are circular and are arranged into a nucleoid consisting of supercoiled DNA and protein.
• Eukaryotic chromosomes are linear.
• It is condensed by being wound around proteins called histones to form nucleosomes, which are then arranged into chromatin fiber. This is further condensed by coiling.
• If the DNA in the cell wasn't well organized, it would be like a huge pile of spaghetti will super long noodles that you can't get apart (or imagine 5 pairs of headphones in your pocket at once... but way, way worse).

Genes

• Genes are sequences of DNA nucleotides. A gene contains the following information:
• The sequence of nucleotides required for the initiation and termination of transcription of the gene (i.e. the promoter and the terminator).
• The sequence of nucleotides that code for a mature RNA molecule, called exons, plus introns (regions that are removed before protein is made).

Chromosomes

• Humans, along with many other species, are diploid, meaning we have two copies of each of our chromosomes (2n).
• This means that our cells contain 23 pairs of chromosomes, giving 46 chromosomes total.
• In humans, one copy of each chromosome comes from your mom, while the other copy of each chromosome comes from your dad.
• Some organisms contain only one copy of each chromosome, and they are said to be haploid (1n).
• It is possible for organisms to have more than two copies of each chromosome. This is called polyploidy.

• The homologous pairs of chromosomes contain the same types of genes, but usually different alleles of each gene.
• An allele can be thought of as a different version of a gene. Example: Let's say chromosome 16 has a gene that encodes for hair color. Each individual has two chromosomes #16, let's call them M and D (for Mom and Dad). At the same location on both chromosomes (locus), there will be a sequence of nucleotides that encodes for a protein that determines your hair color. While on the M chromosome this sequence may be for blonde hair, on the D chromosome it may be for red hair. These two different versions of the same gene are called alleles.
• The sex chromosomes determine if a person will be male or female.
• Two X chromosomes: female.
• One X and one Y chromosome: male.
• Non-sex chromosomes are called autosomes.
• When a cell divides, the chromosomes need to be replicated.
• When the DNA is replicated, each identical copy of a chromosome is called a sister chromatids, and are attached to one another by a centromere.

• After the DNA is replicated, it becomes condensed, and the sister chromatids are attached by the centromere.
• In this state, they two sister chromatids are still considered 1 chromosome.
• When the cell starts to divide into two cells, each sister chromatid is pulled apart. Once separated, they are now considered two chromosomes.