• Chromosomes in eukaryotic cells are?one,?very long?DNA molecule associated with proteins
  • The main proteins present are the large, positively charged globular proteins called?histones, their role is to?organise?and?wrap?the DNA tightly so that it fits into the nucleus
  • The other proteins are?enzymes?used in copying and repairing the DNA
• The tightly coiled combination of DNA and proteins is called?chromatin?– this is what chromosomes are made of

DNA is coiled around histone proteins to form chromatin

The replication of chromosomes

• During?interphase?(the period before mitosis) the DNA replicates to create?two identical strands of DNA called chromatids, joined together by a narrow region called the?centromere
• The two chromatids that make up the double structure of a chromosome are known as?‘sister chromatids’
• It is important that the?sister?chromatids?are?identical?in order to produce?genetically identical?daughter cells via mitosis
  • During?anaphase?of?mitosis?one chromatid ends up in one daughter cell while the other chromatid ends up in the other daughter cell
  • After the centromere is split apart at the start of anaphase the chromatids are referred to as individual?chromosomes?again

Diagram illustrating the structure of a chromosome at different stages of mitosis

• In a eukaryote species, there are different chromosomes that carry different genes
• During mitosis, chromosomes become denser by supercoiling, so?are easier to observe?than when they are in interphase
• Different types of chromosomes can be seen
  • They?differ in length?and the?position of the centromere
• Humans have?23 types of chromosomes
  • The largest one is numbered 1
  • The smallest is numbered 22
  • Pair 23 is the pair of sex chromosomes (XX or XY)
• Humans have between?20,000 and 25,000 genes?across all 46 chromosomes
• Specific?genes always appear at the same locus?(position) of a particular chromosome, for example:
  • The SRY gene found on the Y chromosome causes the development of male genitalia such as the testes
  • The genes that determine eye colour are located on chromosome 15
  • The gene with a faulty version that leads to the disease cystic fibrosis is located on chromosome 7
• In other words, each chromosome type contains?specific genes arranged in a standard sequence
  • This property allows for the exchange of genetic material between chromosomes during meiosis

A pair of homologous chromosomes showing the loci of various alleles

• In?diploid?cells there are two complete sets of chromosomes in the nucleus
• Homologous?chromosomes
  • Carry the?same genes at the same loci
  • Are the?same shape
  • Are?not usually identical?because they may be carrying?different alleles?to each other
• During fertilization, a diploid?zygote?is formed
  • In a zygote, one chromosome of each homologous pair comes from the female gamete and the other comes from the male gamete
• Having the same genes in the same?loci?helps homologous chromosomes line up alongside each other during Metaphase 1 of meiosis
• In photomicrographs, chromosomes are often grouped into their homologous pairs
  • These are shown in a picture format as a?karyogram

Human karyogram showing homologous chromosomes