Robin Burden answered
Frogs' red blood cells each contain a nucleus (unlike those of humans).
In fact, there are no nuclei in the blood cells of any mammal, which is thought to be due to evolutionary mutation.
Why do frogs have nuclei in their red blood cells?
A better question to ask would be, 'why do humans NOT have nuclei in their red blood cells?'.
Based on human evolutionary history, it is likely that we (along with all other mammals) are derived from a species that did, at some point, have nuclei-containing red blood cells.
The fact that we currently don't have nuclei in our red blood cells is probably due to genetic mutation and can, in Darwinian terms, be described as a biological improvement:
It's not all plain sailing for mammals, given their nucleus-free red blood cells, though.
For example, amphibians' red blood cells can reproduce really easily - by simply splitting into two (the DNA contained in the cell's nucleus is used to create new cells).
In humans, red blood cells don't reproduce. Instead, they die every three months, and are sent to the liver, where they are broken down.
The bone marrow is then responsible for creating new red blood cells - a process that is far more taxing on the system than simple cell division.
In fact, there are no nuclei in the blood cells of any mammal, which is thought to be due to evolutionary mutation.
Why do frogs have nuclei in their red blood cells?
A better question to ask would be, 'why do humans NOT have nuclei in their red blood cells?'.
Based on human evolutionary history, it is likely that we (along with all other mammals) are derived from a species that did, at some point, have nuclei-containing red blood cells.
The fact that we currently don't have nuclei in our red blood cells is probably due to genetic mutation and can, in Darwinian terms, be described as a biological improvement:
- Red blood cells that lack nuclei are able to bend and stretch into different shapes more easily, making them able to fit into smaller capillaries.
- Also, the lack of a nucleus also means that each cell has more space to carry oxygen - which is what red blood cells are designed to do within the body.
It's not all plain sailing for mammals, given their nucleus-free red blood cells, though.
For example, amphibians' red blood cells can reproduce really easily - by simply splitting into two (the DNA contained in the cell's nucleus is used to create new cells).
In humans, red blood cells don't reproduce. Instead, they die every three months, and are sent to the liver, where they are broken down.
The bone marrow is then responsible for creating new red blood cells - a process that is far more taxing on the system than simple cell division.