Chromosome Kit: quick revision, 15 bites

Chromosomes: The Messengers of Heredity

Tiny bodies, big inheritance role.

**Chromosomes** are darkly stained, small bodies in the nucleus that determine the mechanism of inheritance. In 1902, **W.S. Sutton** and **T. Boveri** proposed that chromosomes are the physical structures acting as messengers of heredity. They are best seen during **metaphase** of mitotic cell division after specific…

Takeaway. Chromosomes are the physical carriers of hereditary information.

What Exactly Are Chromosomes?

DNA plus proteins equals chromosome.

In eukaryotic organisms, **DNA molecules** in the nucleus combine with proteins, mainly **histones**, to form chromosomes. The makeup of a chromosome is considered to be **highly coiled DNA**. Each species has a characteristic number of chromosomes in each cell.

Takeaway. Chromosomes are highly coiled DNA wrapped around histone proteins.

Chromosome Numbers Vary by Species

Same species, same number.

The number of chromosomes in cells of different species is **variable**. In general, all somatic cells of all individuals of the same species possess the **same number** of chromosomes. Both size and shape of chromosomes differ in different species of fish.

Takeaway. Chromosome number is species-specific and constant within a species.

Human Chromosome Count

46 chromosomes, 23 pairs.

Each cell in every human being has **46 chromosomes** consisting of **23 different pairs**. Of the 23 pairs, one pair determines sex, called **sex chromosomes**. The other 22 pairs are called **autosomes**.

Takeaway. Humans have 46 chromosomes: 22 autosome pairs + 1 sex chromosome pair.

Meiosis Reduces Chromosome Number

Halving for gametes.

The process of **meiosis** occurs during the formation of eggs and sperm. It involves **reduction** of the number of chromosomes to one half. Each gamete receives half as many chromosomes as those contained in the cells of the parent.

Takeaway. Meiosis halves the chromosome number to produce gametes.

Diploid and Haploid Sets

Two sets vs one set.

The chromosome number in body cells is normally made up of **two sets**, one of maternal and one of paternal origin, termed **diploid** and denoted by **2n**. A single set found in a mature egg or spermatozoon is known as a **haploid** set and denoted by **n**. A haploid set of chromosomes is called a **genome**.

Takeaway. Diploid (2n) = two sets; haploid (n) = one set = genome.

Homologous Chromosomes and Genes

One from mom, one from dad.

Because chromosomes occur in pairs, each gene also occurs as a pair. One chromosome of each pair comes from the mother, the other from the father. The two chromosomes that form a pair are called **homologues**. They pair up during the first prophase of meiosis.

Takeaway. Homologues are paired chromosomes, one maternal and one paternal.

Sex Chromosomes and Autosomes

XX-XY defines sex.

In many organisms, a special pair of chromosomes defines sex. In the **XX-XY system** (humans and some fish), females have **XX** and males have **XY**. The other chromosome pairs are called **autosomes**. Some shellfish have no identifiable sex chromosomes, and certain molluscs like oysters may change sex during life.

Takeaway. Sex chromosomes determine sex; autosomes are all other chromosomes.

Polyploidy: Extra Sets of Chromosomes

More than two sets.

When more than two sets of chromosomes occur in cells, the condition is termed **polyploidy**. Many polyploids are **sterile** because their multiple sets segregate irregularly in meiosis. However, polyploids produced by chromosome doubling in interspecific hybrids may be fertile if their constituent genomes segregate…

Takeaway. Polyploidy means extra chromosome sets; often sterile but can be fertile in hybrids.

Euchromatin vs Heterochromatin

Light vs dark staining.

Takeaway. Euchromatin is gene-rich and light; heterochromatin is gene-poor and dark.

Chromosome Structure: Pellicle, Matrix, Chromonema

Membrane, matrix, threads.

Each chromosome structurally consists of a limiting membrane called **pellicle**, an amorphous matrix, and two very thin, highly coiled filaments called **chromonema** or **chromonemata**. The material of chromosomes is called **chromatin**.

Takeaway. A chromosome has a pellicle, matrix, and chromonemata.

Telomeres and Centromeres

Ends and constriction.

Two other features of chromosomes are the ends, called **telomeres**, and a constriction called the **centromere**. Telomeres are stable, usually heterochromatic, and maintain the integrity of chromosome threads. During cell division, microtubules of the spindle attach to centromeres and move them to opposite poles.

Takeaway. Telomeres protect ends; centromeres attach to spindle fibers.

Classification by Centromere Position

Meta, sub, acro.

Takeaway. Centromere position defines chromosome shape and type.

Chromosomes in Fish Culture

Diploids for practical breeding.

For practical fish culture management, fish can be considered **diploids** (chromosomes occur in pairs). Some rare species like **salmonids** and **catostomids** are **tetraploids** (sets of four), but for breeding work they behave as diploids. **Acrocentric** chromosomes are the most common karyotype among fish specie…

Takeaway. Most fish are treated as diploids in breeding, despite some tetraploid exceptions.

Remember the Key Scientists

Sutton and Boveri, 1902.

In 1902, **W.S. Sutton** and **T. Boveri** suggested that chromosomes were the physical structures acting as messengers of heredity. This is a frequently asked fact in exams.

Takeaway. Sutton and Boveri (1902) linked chromosomes to heredity.