UNIT 11 - GENES AND CHROMOSOMES

11.2 Chromosomes

11.2.1 Brief introduction

UNIT 11 - GENES AND CHROMOSOMES

Observations of cells and embryos

in the late 19th century had led to the

recognition that the hereditary

information is carried on

chromosomes, threadlike structures

in the nucleus of a eukaryotic cell

that become visible by light

microscopy as the cell begins to

divide.

→ Eukaryotic cell - A eukaryotic cell has a true membrane-bound nucleus

and has other membranous organelles that allow for compartmentalization of

functions.

UNIT 11 - GENES AND CHROMOSOMES

Later, as biochemical analysis became possible,

chromosomes were found to consist of both

deoxyribonucleic acid (DNA) and protein.

For many decades, the DNA was thought to be

merely a structural element.

However, the other crucial advance made in the

1940s was the identification of DNA as the likely

carrier of genetic information.

→ Deoxyribonucleic acid - A molecule composed of two chains made of

nitrogenous bases (nucleobases) that coil around each other to form a double

helix supported by a sugar-phosphate backbone.

UNIT 11 - GENES AND CHROMOSOMES

→ This complex of nitrogenous bases + deoxyribose sugar + phosphate group

is called a Nucleotide. There’s 5 important kinds nucleotides, differing on the

nitrogenous bases in it (of 2 types, pyrimidines or purines).

Nucleotide Deoxyribonucleic acid

Nitrogenous bases:

Pyrimidines: Cytosine (C), Thymine (T), Uracil (U)

Purines: Adenine (A), Guanine (G)

UNIT 11 - GENES AND CHROMOSOMES

In DNA:

A – T and G – C

In RNA:

A – U and G – C

phosphate group – 5’ end

deoxyribose sugar – 3’ end

5’ end and 3’ end are very important when restriction enzymes are

reading DNA.

UNIT 11 - GENES AND CHROMOSOMES

UNIT 11 - GENES AND CHROMOSOMES

Around 1950s began, both how proteins

could be specified by instructions in the

DNA and how this information might be

copied for transmission from cell to cell

seemed completely mysterious.

The mystery was suddenly solved in

1953, when the structure of DNA was

correctly predicted by James Watson and

Francis Crick and their super famous double-helix.

The double-helical structure of DNA immediately solved the problem of

how the information in this molecule might be copied or replicated.

It also provided the first clues as to how a molecule of DNA might use the

sequence of its subunits to encode the instructions for making proteins.

UNIT 11 - GENES AND CHROMOSOMES

UNIT 11 - GENES AND CHROMOSOMES

11.2.2 Structure

DNA - A molecule composed of two coiled chains of nucleotides (A,

G, C, T)

Histone - proteins found in eukaryotic cell nuclei that package and

order the DNA into structural units.

Nucleosomes - These packed units are called nucleosomes.

Chromatin - Chromatin is a complex of DNA, RNA and protein with

the primary function of packaging very long DNA molecules into a

more compact denser shape

Preventing the strands from becoming tangled

Reinforcing the DNA during cell division, preventing DNA

damage

Regulating gene expression and DNA replication.

UNIT 11 - GENES AND CHROMOSOMES

UNIT 11 - GENES AND CHROMOSOMES

Chromosome – During nuclear division, such as mitosis, each of these

fibers coils up to form a tightly-packed, long, thread-like structure

called chromosome

Chromosomes can be seen through a light microscope when cells are

dividing. Watch this video:

https://www.youtube.com/watch?v=L61Gp_d7evo

UNIT 11 - GENES AND CHROMOSOMES

11.2.3 Diploid and Haploid Number

Diploid number – Every species has its own specific amount of

chromosomes (humans 46, potatoes 48, etc.) per cell.

This amount of chromosomes is called the diploid number (2n), that is,

the number of pair of chromosomes (2*23 = 46 in humans).

Haploid number – When germ cells undergo meiosis, the 2 pairs of

chromosomes split, becoming the genetic material of 2 daughter cells with

a haploid number (n) of chromosomes (23 in humans).