Genes and Inheritance [2]: Genes, Chromosomes and Alleles (A Level Biology)

Uploaded by freeeschool on 02.09.2012

A Level Biology: Genes and Inheritance 2 - Genes, Chromosomes and Alleles
Hi! Welcome to my second video about genes and how they are organized within the cell.
The last time, we finished by looking at that smallest sub-unit of genetic material which
is the DNA and how lots of pieces of DNA put together form a gene. So a gene is a section
of DNA that controls a particular characteristic. An example of that is eye color or hair color.
On the section here, you have two genes that have been labeled. Now in reality, a gene
is going to be much, much larger than this. On average, this seems to show that we have
roughly between 12 to 15 nucleotide bases that make up a gene. In reality, a gene is
usually made up of thousand of nucleotide bases.
This structure you see on the middle here is what is known as a chromosome and specifically,
this is a pair of chromosomes joined together. You notice that how basically two sections
and is joined around this middle part here. These different bandings that you get here,
these represent different genes. So, on a given chromosome you can have several thousands
in gene. In fact, the total of human genome is we have roughly 23,000 genes.
This section of DNA basically what happens is it is coiled up and it forms of shape in
the chromosome. If we could magnify on these sections here, then what we’d see is a lot
of the double helix and lot of pieces of DNA, tend to be folded in on itself and therefore,
they form these different structures known as chromosomes.
There is such a variety in the human population that obviously, not all genes are the same.
These genes can come in various different types or different version of a gene which
is sometimes referred to as an allele. Generally speaking, you have two alleles that represent
a single gene. We will come on to look at the relationship between these alleles in
a later video.
Inside a given nucleus inside a given cell, you have 23 pairs of these chromosomes. The
reason why these chromosomes are referred to as 23 pairs and not 46 is, as you can see
here, these are the 23 chromosomes that you possess and each one of them comes in a pair.
So 23 x 2 = 46 and you receive each chromosome from a single parent. For example, this one
may have come from your mom and the other one will come from your dad.
The only one that is an exception to this is the 23rd chromosome which is the sex chromosome.
You see down here, it comes in the form of X or Y. Essentially, on the 23rd chromosome,
what you’ll have if you are a female is XX and if you are male, you have XY.
This diagram shows a typical cell with a typical nucleus that contains 23 pairs or 46 chromosomes,
of which you have roughly between 23,000 to 24,000 genes and roughly 3 billion pieces
of DNA.
In summary, you have genes which are codes for characteristics. Again, this might be
something like eye color or hair color.
Different versions of a gene are referred to an allele. We usually have two alleles
per gene. The DNA will fold in itself and compact itself into these shapes called chromosomes,
of which you have 23 pairs of those, but sometimes referred to as 46. This is true for every
cell in your body with the exception of sex cells and red blood cells.
[end of audio – 03:57] A Level Biology: Genes and Inheritance 2 – Genes,
Chromosomes & Alleles Page…1