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).