Asked by SarahBiology 🌱

A cell enters in meiosis after interphase with 20 picogram DNA. Why is it said that only one-fourth DNA will be present in each gamete produced after meiosis (when we know that the chromosome number is just halved)?

Profile picture

VERIFIED

Verified tutor tick

Nathan Rawle

Oxford Master of Biochemistry - boss of Biology, Chemistry, Physics, and Maths

The cell starts with two copies of every chromosome: one paternal and one maternal. After interphase but before the cell divides every chromosome is duplicated. At this point, the cell has *four* copies of every chromosome: two identical paternal copies and two identical maternal copies (these form the “X”-shaped structures that chromosomes are usually depicted as). After homologous recombination, the cell goes through the first meiotic division (meiosis I) producing two daughter cells. One daughter cell gets the recombined paternal copies and the other daughter cell gets the recombined maternal copies. Both daughter cells then go through the second meiotic division (meiosis II) resulting in four “grand-daughter” cells (gametes). Each grand-daughter cell gets one of the recombined copies from the original “grand-mother” cell: two of the grand-daughter cells each get one of the recombined paternal copies and the other two grand-daughter cells each get one of the recombined maternal copies. Since the DNA mass is split equally (and no more synthesis occurs) the amount of DNA is halved during each meiotic division. Since the mother cell undergoes meiotic division twice after chromosome duplication, each grand-daughter cell contains 1/4 of the DNA that was present in the mother cell right before it entered meiosis (a half of a half = a quarter). N.B.: the cells go from diploid to haploid during meiosis I. The mother cell is diploid. All of the daughter and grand-daughter cells are haploid. The “chromosome number” is not changed by duplication nor by meiosis II. This can be confusing, so ask your teacher to explain the difference between a chromosome and a “chromatid”. Before duplication, each chromosome exists as a single chromatid. After duplication, each chromosome exists as a pair of “sister” chromatids. The number of chromosomes stays the same, but the number of chromatids has doubled. Meiosis I is the separation of chromosomes (as chromatid pairs) into the daughter cells and meiosis II is the separation of chromatids into the grand-daughter cells. It might help to draw out this whole process, starting from a diploid cell in interphase with only one pair of (single-chromatid) chromosomes (one maternal and one paternal). p.s.: I think the question is a little bit misleading, since the total amount of DNA in each gamete is only *half* that present in the original (grand-mother) cell *during* interphase. You can only say that each gamete has a quarter of the DNA of the original cell if you’re starting from the amount of DNA *after* duplication, i.e. after interphase has finished and right before meiosis I. This is a weird way to think about ploidy, in my opinion!

Nathan also answered

Asked in Biology 🌱

ASKED BY VICKY

BIOLOGY 🌱

How do i get clearer skin?

Increase your water intake try to start with half a litre to 1 litre at first so that you don’t get tempted to not stick with it. Cut down on the ju...

ASKED BY KYLE

BIOLOGY 🌱

How are epithelial cells effected by COPD?

simply speaking there is more of inflammation and mucus production. there is a whole mechanistic pathway involved in the process of cells being effe...

ASKED BY EMMA

BIOLOGY 🌱

GCSE Biology- What is the difference between nitrifying bacteria, denitrifying bacteria, nitrifying-fixing bacteria and decomposing the bacteria, in regards to the nitrogen cycle?

Nitrifying bacteria: converters of soil ammonia to nitrates, compounds usable by plants. Denitrifying bacteria in the soil break down nitrates and r...

ASKED BY AUDREY

BIOLOGY 🌱

How does pulse differ from blood pressure?

Hi Audrey, good question. Your blood pressure is a measurement of the force the blood is being passed through the blood vessels. It is measured as s...

ASKED BY AUDREY

BIOLOGY 🌱

If neurons in the central nervous system lack centrioles and are unable to divide, how do brain tumors develop?

Hi Audrey, There are more than one type of cell in the brain, these include astrocytes, microglia and other structural cells. Whilst the neurons do ...

ASKED BY MAX

BIOLOGY 🌱

The vaccine for tuberculosis contains a dead form of the tuberculosis bacterium. how does the body respond to the vaccine? How is passive immunity brought about? When is it needed?

There are several different types of vaccine, dead, attenuated and acellular. The idea is the the vaccines contain proteins/lipids that the immune s...

ASKED BY CHENCHEN

BIOLOGY 🌱

What is mitosis and how does it work?

Hi Chenchen, mitosis is the type of cell division that results in a new cell with the same amount of chromosomes as the original cell. Essentially t...

ASKED BY SAN SAN

BIOLOGY 🌱

What is biodiversity?

the variety of plant and animal life in the world or in a particular habitat, a high level of which is usually considered to be important and desira...

ASKED BY RAPHAEL

BIOLOGY 🌱

How does the plant use glucose?

The most basic answer to this is to provide energy. Like many living things plants use glucose in a chemical reaction called respiration that uses o...

ASKED BY CHENCHEN

BIOLOGY 🌱

What are stem cells?

Stem cells are non specialised cells, which as well as dividing to produce more cells as all other cells do, can differentiate in any other type of ...

Find me a tutor

We take your privacy seriously. View our policy.