Posted on

Sister Chromatids and Non-sister Chromatids – What is the Difference?

Sister Chromatids and Non-sister Chromatids – What is the Difference?

It is one of those posts where we clarify doubts between confusing terms. I am talking about sister chromatids and non-sister chromatids. When a cell undergoes cell division, newly produced daughter cells should get the copies of complete set of chromosomes. For this purpose, all the chromosomes undergo replication and produce its identical copy.

  • Sister Chromatids –

When chromosomes are replicated they are attached at the centromere before these replicated copies are separated. So these identical copies of chromosome are called sister chromatids. They are called sister chromatids because all the genes and to be specific all the alleles are exactly the same because they are identical copies  (Figure 1).

Sister Chromatids = Replicated Chromosomes Attached at the Centromere.

Figure 1. Sister Chromatids

  • Non-sister Chromatids –

We are diploid organisms which means for each pair of chromosome we have one chromosome coming from mother and one chromosome coming from father. Therefore we have homologous pairs for every chromosome.

Figure 2. Non-sister Chromatids

Let’s take figure 2 in consideration and let’s say it is chromosome 1. So it will have its homologous pair and that homologous chromosome also replicates. This would be true for every homologous pair. For this homologous chromosome all the genes are same but alleles might be different (because one has come from mother and one has come from father). Therefore in a homologous pair, the chromosome copies are not identical, isn’t it? Even though the genes are same but the alleles might be different. And because these copies are not exactly the same they are called non-sister chromatids.

Non-sister Chromatids = Replicated Chromosomes of Homologous Pair. 

I hope this post was helpful 🙂

Watch a video on this topic here.

 

Posted on

Genes vs Alleles – What is the Difference?

Genes vs Alleles – What is the Difference?

In this post we will see what are genes and alleles and the difference between them. Genes and Alleles are two terms we use so frequently in genetics. To understand these two terms we would have to start with our DNA. DNA is our genetic material and it is polymer of nucleotides which codes for all our physical characteristics, proteins etc. Here we are saying that DNA codes for all these but what exactly does that mean?

Say for example, a patch of DNA containing a particular nucleotide sequence codes for eye color. This patch of DNA is called GENE (figure a). Likewise there can be many genes in DNA which codes for various characteristics, proteins or enzymes etc.

a. gene

  • Therefore, GENE = a patch of DNA containing specific nucleotide sequence that codes for something.

Now let us go to chromosome and see the same gene on chromosome. We talked about a gene coding for eye color, just for an example assume that this gene is present on chromosome number 1. We have 23 pairs of chromosomes meaning every chromosome has a pair so here chromosome 1 also has a homologous pair. In this homologous pair also gene coding for eye color is present in the same locus as shown in figure b. This point is very important to understand that the gene location is always specific on chromosomes meaning at whichever locus the gene for eye color is present at in chromosome 1 is same for the homologous chromosomes also. This means we contain two copies of the same gene, isn’t it! And each copy in this gene is called an ALLELE. So we have two alleles of the same gene.

b. Alleles

Now let us understand one more point. See both the copies of genes are same meaning it has function of making eye pigmentation but in both the copies of the genes there can be minute difference in the nucleotide sequence which makes it code for different pigmentation or variation. For example, we saw chromosome 1 has two alleles for eye color gene, one of these alleles code for black pigmentation and the other alleles on homologous chromosome codes for brown pigmentation. And you will see phenotype of whatever alleles is dominant.

  • Therefore, ALLELE = variation of the same gene.

In a population there can be multiple alleles. That is how we have different eye colors such as black, light brown, dark brown, green, blue etc. same for different hair colors and all other physical traits that we see. But in an individual we can have only two alleles present for any particular given gene and that is because we carry homologous pairs of every chromosome. Each chromosome in that pair carries that particular allele; since we have two chromosomes in each pair we can have only two alleles of any particular gene.

Hope I am able to make it clear to you guys 🙂

 
Watch this video for better understanding.