How Many Chromosomes Do Humans Have?

What Are Chromosomes?

Each cell in the human body contains a nucleus within, which possesses DNA molecules that are packaged as a thread-like structure called Chromosomes. Chromosomes are made using DNA that is coiled multiple times around a protein called histones, which is responsible for supporting its structure.

If a cell is not in the process of dividing, the chromosomes are not visible at all, not even under a microscope. However, when a cell is dividing that is when chromosomes become visible under a microscope since that is when the DNA gets much more tightly wrapped. Hence, everything that researchers know today about a chromosome was actually learned through conducting observations during the process of cell division.

A chromosome is divided into two sections, also known as the two “arms”; and the centromere is the constriction point for each chromosome that causes this division. Both these arms are differently sized, so the shorter one is called the “P arm”, while the longer one is called the “Q arm”

What Are Homologous Chromosomes?

Homologous chromosomes are basically two human chromosomes that are paired together – generally inherited from both parents, one from each. For instance, 2 copies of Chromosomes within a single cell will be known as homologous chromosomes.

In terms of biology, homologous chromosomes are paired human chromosomes. The similarities are that they possess identical centromere location, loci (gene position), gene sequence, and chromosomal length. Having said that, even if their genetic sequence and loci are exactly the same; there will definitely be differences in alleles. 

How Many Chromosomes Do Humans Have?

Chromosomes are always found to be paired with one another – and there are 23 pairs of human chromosomes for each cell present in our body, which makes it a total of 46 chromosomes. This total number of human chromosomes is inherited, since half of these chromosomes come from the father, while the other half come.

What Are Chromosomes Made Of?

Being an intertwined structure found inside the nucleus of a cell – A chromosome is made up of proteins and DNA that are organized into genes. And each cell normally contains 23 pairs of chromosomes.

Where In The Cell Are Chromosomes Located?

Found in the nucleus of a cell, chromosomes are structures that are responsible for carrying long pieces of DNA. Being the building block and the base for the human body, DNA is responsible for holding genes together.

Homologous Chromosomes Vs Sister Chromatids

It is essential to understand the difference between sister chromatids and homologous chromosomes. Sister chromatids have their utilization in cell division such as cell replacement. While homologous chromosomes are capable of being used in the reproductive division, to help make new babies.

Sister chromatids are genetically the same, being identical copies of one another specifically created for cell division. In fact, the term sister chromatid is only used during the parts of cell division when the structures are in that X shape, or when a centromere connects the two copies.

Also, a pair of homologous chromosomes consists of two non-identical copies of a chromosome, one from each parent. Here are a few more differences between sister chromatids and homologous chromosomes:

– Function

Homologous chromosomes allow random segregation of chromosomes and genetic recombination during metaphase I.

Sister chromatids allow random segregation of chromatids and chromosomal crossover during metaphase II of meiosis and metaphase of mitosis.

– Joining

Homologous chromosomes do not stick together, they exist in pairs. Sister chromatids are joined together by their centromere.

– Genetic Composition

Homologous chromosomes may contain the same or different alleles of the same gene. Thus, the gene sequence is not identical at all times.

Sister chromatids contain identical gene sequences throughout the chromatids except in the chromosomal crossover.

– Appearance

Homologous chromosomes appear in metaphase I of meiosis I. Sister chromatids are formed during the DNA replication in the S phase of interphase.

– Number of DNA Strands

Homologous chromosomes are composed of four DNA strands.

A single sister chromatid is composed of a single DNA strand.

– Separation

Homologous chromosomes are segregated during anaphase I of meiosis I.

Sister chromatids are separated from their centromere during anaphase II of meiosis II and the anaphase of mitosis.

During Which Three Phases Are Individual Chromosomes No Longer Visible?

There are three phases during which the chromosomes are no  longer visible:

i) Interphase
ii) Cytokinesis
iii) Telophase 

Interphase is the stage of the cell cycle during which a cell becomes prepared to be divided. It is the longest phase of the cell cycle that takes almost 23 hours to complete in human cells.

In telophase, the chromosomes decondensed back after the nuclear division has occurred.

And in cytokinesis also, the cytoplasmic division takes place after telophase and therefore, the chromosomes are not visible.

– Disclaimer –
This blog is for informational & educational purposes only, and does not intend to substitute any professional medical advice or consultation. For any health related concerns, please consult with your physician, or call 911.