Genes, chromosomes, base pairs and the 23 pairs

OK so I'm a bit confused. Does a chromosome consist of genes and do genes consist of DNA? What about the 23 pairs in sexual reproduction? I can't remember what the pairs consist of

I will start from the small and go to the big organisational unit: All of this is constructed by DNA, the sequence of the four bases: adenosine, thymidine, cytidine and guanosine. They are abbreviated G, A, T and C and are called the letters of DNA.

Genes are organisational units formed by a specific sequence of the four "letters". Genes encode for all the proteins (in fact this is a simple view, but should be enough for the moment) and are the way the information of how the protein sequence looks is conserved.

All DNA is organised in the form of a long string of bases, which would be very long in higher organisms. To come around this problem, the DNA is organised in chromosomes, in which the DNA is wound around special proteins (the histones) and further wound up to get a compact form. The image below (from here) shows the relationship:

The 23 chromosome pairs are the 23 organisational units in humans, in which our genetic information is stored and organised. A picture of them looks like this (image from the same website as above):

Please note that the chromosomes are not X-shaped, although they might look like on the image. They are pairs of long DNA molecules, which are not connected.

6.3: Chromosomes and Genes

  • Contributed by Suzanne Wakim & Mandeep Grewal
  • Professors (Cell Molecular Biology & Plant Science) at Butte College

Identical Twins, Identical Genes

You probably can tell by their close resemblance that these two individuals are identical twins. Identical twins develop from the same fertilized egg, so they inherited copies of the same chromosomes and have all the same genes. Unless you have an identical twin, no one else in the world has exactly the same genes as you. What are genes? How are they related to chromosomes? And how do genes make you the person you are?

Figure (PageIndex<1>): Identical twins

Chromosomes are coiled structures made of DNA and proteins. Chromosomes are encoded with genetic instructions for making proteins. These instructions are organized into units called genes. Most genes contain the instructions for a single protein. There may be hundreds or even thousands of genes on a single chromosome.

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The race to sequence the human genome – Tien Nguyen

Chromosomes and Genes

Each species has a characteristic number of chromosomes. The human species is characterized by 23 pairs of chromosomes, as shown in Figure below and Figure below. You can watch a short animation about human chromosomes at this link:

What are Chromosomes (Bozeman Science):

Chromosome Numbers During Division (Amoeba Sisters)

Try filling out this printable after watching the video above:

Of the 23 pairs of human chromosomes, 22 pairs are autosomes (numbers 1–22 in Figure above). Autosomes are chromosomes that contain genes for characteristics that are unrelated to sex. These chromosomes are the same in males and females. The great majority of human genes are located on autosomes. At the link below, you can click on any human chromosome to see which traits its genes control.

Sex Chromosomes

TED Ed: Sex Determination: More Complicated Than You Thought
Note: there is a brief mention of evolution.

The remaining pair of human chromosomes consists of the sex chromosomes, X and Y. Females have two X chromosomes, and males have one X and one Y chromosome. In females, one of the X chromosomes in each cell is inactivated and known as a Barr body. This ensures that females, like males, have only one functioning copy of the X chromosome in each cell. As you can see from Figure above and Figure above, the X chromosome is much larger than the Y chromosome. The X chromosome has about 2,000 genes, whereas the Y chromosome has fewer than 100, none of which are essential to survival. Virtually all of the X chromosome genes are unrelated to sex. Only the Y chromosome contains genes that determine sex. A single Y chromosome gene, called SRY (which stands for sex-determining region Y gene), triggers an embryo to develop into a male. Without a Y chromosome, an individual develops into a female, so you can think of female as the default sex of the human species. Can you think of a reason why the Y chromosome is so much smaller than the X chromosome?

Human Genes

Humans have an estimated 20,000 to 22,000 genes. This may sound like a lot, but it really isn’t. Far simpler species have almost as many genes as humans. However, human cells use splicing and other processes to make multiple proteins from the instructions encoded in a single gene. Of the 3 billion base pairs in the human genome, only about 25 percent make up genes and their regulatory elements. The functions of many of the other base pairs are still unclear. To learn more about the coding and noncoding sequences of human DNA, watch the animation at this link:

The majority of human genes have two or more possible alleles. Differences in alleles account for the considerable genetic variation among people. In fact, most human genetic variation is the result of differences in individual DNA bases within alleles.


Genes that are located on the same chromosome are called linked genes. Alleles for these genes tend to segregate together during meiosis, unless they are separated by crossing-over. Crossing-over occurs when two homologous chromosomes exchange genetic material during meiosis I. The closer together two genes are on a chromosome, the less likely their alleles will be separated by crossing-over.

Linkage explains why certain characteristics are frequently inherited together. For example, genes for hair color and eye color are linked, so certain hair and eye colors tend to be inherited together, such as blonde hair with blue eyes and brown hair with brown eyes. What other human traits seem to occur together? Do you think they might be controlled by linked genes?

Sex-Linked Genes

Genes located on the sex chromosomes are called sex-linked genes. Most sex-linked genes are on the X chromosome, because the Y chromosome has relatively few genes. Strictly speaking, genes on the X chromosome are X-linked genes, but the term sex-linked is often used to refer to them.

Mapping Linkage

Linkage can be assessed by determining how often crossing-over occurs between two genes on the same chromosome. Genes on different (nonhomologous) chromosomes are not linked. They assort independently during meiosis, so they have a 50 percent chance of ending up in different gametes. If genes show up in different gametes less than 50 percent of the time (that is, they tend to be inherited together), they are assumed to be on the same (homologous) chromosome. They may be separated by crossing-over, but this is likely to occur less than 50 percent of the time. The lower the frequency of crossing-over, the closer together on the same chromosome the genes are presumed to be. Frequencies of crossing-over can be used to construct a linkage map like the one in Figure below. A linkage map shows the locations of genes on a chromosome.

Lesson Summary

  • The human genome consists of about 3 billion base pairs of DNA. In 2003, the Human Genome Project finished sequencing all 3 billion base pairs.
  • Humans have 23 pairs of chromosomes. Of these, 22 pairs are autosomes. The X and Y chromosomes are the sex chromosomes. Females have two X chromosomes, and males have one X and one Y. Human chromosomes contain a total of 20,000 to 22,000 genes, the majority of which have two or more alleles.
  • Linked genes are located on the same chromosome. Sex-linked genes are located on a sex chromosome, and X-linked genes are located on the X chromosome. The frequency of crossing-over between genes is used to construct linkage maps that show the locations of genes on chromosomes.

Lesson Review Questions


1. Describe the human genome.

2. What has the Human Genome Project achieved?

4. Describe human genetic variation.

Apply Concepts

5. Explain how you would construct a linkage map for a human chromosome. What data would you need?

Think Critically

6. Compare and contrast human autosomes and sex chromosomes.

7. People with red hair usually have very light skin. What might be a genetic explanation for this observation?

Points to Consider

You read in this lesson about the chromosomes and genes that control human traits. Most traits are controlled by genes on autosomes, but many are controlled by genes on the X chromosome.

What is a Chromosome?

In the nucleus of each cell, the DNA molecule is the chemical unit of inheritance, and a long polymer of DNA forms a gene, and when many of such genes get packaged (supercoiled/coiled) into a bigger thread-like structure forms a chromosome.

Each chromosome is made up of DNA tightly coiled many times around proteins called histones that support its structure.

DNA is so compressible that a DNA helix with a diameter of 2nm (2 x 10 -9 m) can be supercoiled to become a chromatid of 700nm (700 x 10 -9 m) diameter or so. A chromosome consists of two chromatids attached together.

Chromosomes contain various genes that carry various genetic information as a whole.

The genes on each chromosome are arranged in a particular sequence, and each gene has a particular location on the chromosome called its locus.

In each human diploid cell, there are 23 pairs (46 nos.) chromosomes. One such human Y chromosome contains over 200 genes, at least 72 of which code for proteins. These 200 genes can have about 58 million base pairs of DNA.

DNA, Chromosomes and Genes

DNA is called the blueprint of life because it contains the instructions needed for an organism to grow, develop, survive and reproduce. DNA does this by controlling protein synthesis. Proteins do most of the work in cells, and are the basic unit of structure and function in the cells of organisms.

Genes are the basic units of heredity and are located on chromosomes.

Genes are sections of DNA, whereas chromosomes are the structures that DNA folds into before cell division. Each human somatic cell contains 23 pairs of chromosomes. All of the genes that code for the creation, growth, and development of a human person are found in these chromosomes. In addition to DNA, these chromosomes contain histone proteins that help in the packaging of the DNA into chromosomes.

In eukaryotic cells, chromosomes are found in the nucleus but in prokaryotic cells they are free to move about.


Deoxyribonucleic Acid is the meaning of it. It is a nucleic acid that is the carrier of genetic information.


DNA are the letters of deoxyribonucleic acid.

All life on earth uses this nucleic acid as the genetic code.

A nucleic acid is a polynucleotide. A polynucleotide consists of three basic units: a phosphate group, a 5 carbon sugar (pentose), and a nitrogenous base. The five carbon sugar is deoxyribose. Since a polynucleotide chain, the phosphate and deoxyribose units are repetitive, the variation is provided by the nitrogenous bases.

There are four bases: adenine, guanine, cytosine, and thymine.
Both adenine and guanine are purines which have a double ring structure. Cytosine and thymine are pyramidines which consist of a single ring structure.

The DNA molecule is double helix, a spiral shaped ladder. The upright or backbone of the ladder is made of alternating pentose and phosphate groups held together by covalent bonds. The rungs or steps of the ladder consist of the bases. These bases are joined to the pentose sugars with covalent bonds. Adenine pairs with thymine using two hydrogen bonds and cytosine pairs with guanine using three hydrogen.

The genetic code is determined by the linear sequence of the bases.

For example the sequence of adenine guanine thymine does not carry the same message as guanine thymine adenine.

The code is arranged in triplet form which codes for RNA which in turn codes for amino acids which form the basis of proteins.

A gene is a distinct portion of your cell’s DNA. Genes are coded instructions for making everything your body needs, especially proteins. You have about 25,000 genes. Researchers have yet to determine what that majority of our genes do, however, some of our genes can be associated with disorders such as cystic fibrosis or Huntington’s disease.

Proteins are chains of chemical building blocks called amino acids. A protein could contain just a few amino acids in its chain or it could have several thousand. Proteins form the basis for most of what your body does such as digestion, making energy, and growing.


The gene is the basic physical and functional unit of heredity. It consists of a specific sequence of nucleotides at a given position on a given chromosome that codes for a specific protein (or, in some cases, an RNA molecule).

Genes consist of three types of nucleotide sequence:

  • coding regions, called exons, which specify a sequence of amino acids
  • non-coding regions, called introns, which do not specify amino acids
  • regulatory sequences, which play a role in determining when and where the protein is made (and how much is made)

The structural components of a gene

A human being has 20,000 to 25,000 genes located on 46 chromosomes (23 pairs). These genes are known, collectively, as the human genome.

Genes, chromosomes, base pairs and the 23 pairs - Biology

I’m sorry to burst your bubble, but you don’t have a gene for being tall, or a gene for caffeine addiction or a gene for intelligence. No one does. In an effort to simplify the science, the media sometimes talks about genetics and the influence of genes in these terms.

But there is not a “breast cancer gene,” or a “warrior gene.” It’s much more complicated than that. With a few exceptions, everyone has the same genes. But within these genes, there are slight variations – different genotypes – and it’s those small variations that makes us all unique.

Technically speaking, the answer is always:

“Yes.” The thing is, that’s not really what they are asking. They want to know if they have the genotypes that influence a certain trait. To understand what I mean by genotype, it helps to review a little biology. Think of your DNA as one long string. A very, very long string that’s crammed into every cell in your body. This string is broken into many segments with each segment serving a different purpose. These segments of DNA are called genes.

Genes provide instructions for your body to produce different proteins. Proteins influence nearly every function of your body. Some proteins effect visible traits like the color of your eyes. Others prompt biological functions from the ability to lift your arm to intricate cell processes. With only a few exceptions, everyone has the same genes, so it’s not the presence or absence of a gene that is variable.* It’s what is within those genes that is different. Each gene contains many positions, and each position is associated with a single genotype. It is the genotypes on a particular gene that make all the difference. Let’s break this down a bit more.

Your DNA is made up of nucleotide bases – adenine, cytosine, guanine, and thymine. Your A’s, C’s, G’s and T’s, if you will. You can think of these bases like puzzle pieces. They bind with one another to form your DNA. Your DNA is organized into twenty-three pairs of chromosomes. These bases together make up your genotype at that position. A genotype is written using the two letters of the two nucleotide bases at a particular position, such as AA or CT. There are several ways scientists refer to particular points in your DNA.

At 23andMe, we perform genotyping by analyzing single nucleotide polymorphisms (SNPs). It is a location on your DNA where the genotype can vary among people. This means that your genotype at that position may differ from other individuals. Different genotypes at a particular SNP can indicate something about your physical characteristics, or potential genetic health risk. Genotyping services base interpretations on your genotype for a particular SNP or SNPs, not the presence or absence of a gene.

For example, the type of earwax you have is dependent on a single SNP located on the ABCC11. If your genotype at this SNP is TT, then you have dry earwax. If you have any other genotype at this SNP, then you have wet earwax. And that’s it! So the next time you wonder if you have a particular gene, you’ll know it’s a bit more complicated than that. It’s all about those base pairs. ___

*There are some conditions caused by gene deletions. 23andMe is not well-suited to detect such genetic changes.

Genetics (Science 1.9)

Welcome to the topic of genetics! Genetics is the study of heredity. Heredity is a biological process where a parent passes certain genes onto their children or offspring. Every child inherits genes from both of their biological parents and these genes in turn express specific traits. Some of these traits may be physical for example hair and eye color and skin color etc. On the other hand some genes may also carry the risk of certain diseases and disorders that may pass on from parents to their offspring. This topic is like a new language for most so stick at it. Learn the key words, watch the videos and listen to your teachers!

Fantastic PowerPoint and resources from GZScience online here

Before you start work through the following activities from DNA from the beginning.

No brain to small - This website has some amazing!! resources - go have a look here

Below are the key parts to know:

Nucleus is the Organelle in a cell that contains DNA.

Deoxyribonucleic Acid is a self-replicating molecule present in nearly all living organisms. It’s what the chromosomes are made up of.

The Chromosome is a threadlike structure of nucleic acids and protein found in the nucleus of most living cells. They carrying genetic information in the form of genes. Chromosomes are made up of long lengths of DNA.

Chromosomes Location: Contained within the nucleus

Made up of: DNA (nucleic acids – a phosphate, sugar and base) with various binding proteins holding it together

Function (what it does): Containing genetic information to enable an organism to manufacture all the proteins required to develop and maintain an organism when necessary.

A Gene is a short length of DNA that carries the genetic code for a particular characteristic or cell activity. Different forms of the same gene are called Alleles. They can be dominant or recessive.

A trait is a genetically determined characteristic such as eye colour or hair colour.

Benjamin Himme's epic video on DNA, she is a long one!

DNA is the heredity material of the cell which is found in the chromosomes in the nucleus. These are found as strands each one of these strands of DNA is called a chromosome. A gene is a segment of DNA, found in a small section of the chromosome. Along the DNA, base sequences provide the code for building different proteins, which then determine particular features. Slight differences in the sequence of the bases making up a gene are called alleles and they cause the variations in the phenotypes. These differences lead to genetic variation between individuals.

From 2013's exam.. Chromosomes are made up of DNA. DNA is a large molecule that is coiled into a double helix (twisted ladder structure). It is responsible for determining the phenotype of an organism. Along this molecule are bases. These bases pair up A always pairs with T, and G with C.

A sequence of bases which codes for a particular trait (eg, eye colour) is called a gene.

The different versions of each gene are called alleles, and these show the different variations of each characteristic, eg brown / blue eyes. Because chromosomes come in pairs for each trait, there will be two possible alleles. These different versions of genes (alleles) occur as the DNA base sequence is different.

This combination of alleles for each trait is called the genotype this can be any combination of two of the available alleles. The genotype determines the phenotype (the physical appearance) of the organism. Whichever alleles are present may be expressed. Dominant alleles (B) will be expressed over recessive alleles (b).

DNA from the Beginning Good introduction to DNA and genetics

DNA, or deoxyribonucleic acid, is the hereditary material in humans and almost all other organisms. Nearly every cell in a person’s body has the same DNA. Most DNA is located in the cell nucleus (where it is called nuclear DNA), but a small amount of DNA can also be found in the mitochondria (where it is called mitochondrial DNA or mtDNA).

The information in DNA is stored as a code made up of four chemical bases: adenine (A), guanine (G), cytosine (C), and thymine (T). Human DNA consists of about 3 billion bases, and more than 99 percent of those bases are the same in all people. The order, or sequence, of these bases determines the information available for building and maintaining an organism, similar to the way in which letters of the alphabet appear in a certain order to form words and sentences.

DNA bases pair up with each other, A with T and C with G, to form units called base pairs. Each base is also attached to a sugar molecule and a phosphate molecule. Together, a base, sugar, and phosphate are called a nucleotide. Nucleotides are arranged in two long strands that form a spiral called a double helix. The structure of the double helix is somewhat like a ladder, with the base pairs forming the ladder’s rungs and the sugar and phosphate molecules forming the vertical sidepieces of the ladder.

The shape of DNA at the molecular level is thought to look like a gently twisting ladder. Each of the rungs on the ladder represents a chemical bond between the chemicals that make up the DNA molecule. These chemicals are called nucleotides and include: (click on image to make bigger)

DNA is made from Deoxyribonucleic Acid. DNA is called a polymer because it is made up of many repeating units called nucleotides.

DNA strands are loose within the nucleus of a cell. Just prior to cell division the DNA folds up around proteins called histones into tight coils, then into structured chromosomes. The human cell has 46 chromosomes arranged into 23 pairs of chromosomes. Each chromosome in a pair has the same genes, called homologous pairs – except the sex chromosome pair – although there may be variation between the genes of each pair, as one comes from the father and one comes from the mother.

Chromosomes are made up of long lengths of DNA arranged in a twisted ladder.

The base pairing rules mean that guanine (G) always bonds to cytosine (C), and thymine (T) always bonds to adenine (A).

A gene is a section of DNA that carries the genetic code for a particular characteristic. An allele is an alternative form of a gene. They can be dominant or recessive.

During fertilisation a person gets two different alleles for the same gene because one allele is inherited from your mother, the other comes from your father.

A great video that covers this in more detail than is required a L1. You need to know where it happens, why it happens and the basics steps that are below.

The original DNA strand unwinds as the bonds between the bases break.

New nucleotides are brought in. They bond with the bases on the original DNA strand according to the base-pairing rules.

Once the new nucleotides have bonded, the DNA molecule begins to coil back up into a double helix. At the end of the process, two new strands of DNA are produced. Both are exact copies of the original strand.

Chromosomes come in pairs. One pair is the sex chromosomes – XX in females and XY in males. A complete set of chromosomes of an organism placed into pairs of matching chromosomes is called a karyotype. The human karyotype consists of 23 pairs of chromosomes

Genotype is the genetic make-up of an individual organism. Your genotype functions as a set of instructions for the growth and development of your body. The word ‘genotype’ is usually used when talking about the genetics of a particular trait (like eye colour).

Phenotype is the observable physical or biochemical characteristics of an individual organism, determined by both genetic make-up and environmental influences, for example, height, weight and skin colour.

A codon is a group of three bases that code for an specific amino acid.DNA contains the instructions for linking amino acids. These amino acids join together to make proteins. Proteins are important because they are the building blocks of our body and carry out many important functions within the body. The base sequence of DNA can be broken down into codons (three-letter sequences). One codon codes for one amino acid.

Homologous pairs are chromosomes that have the same genes.

Biological concepts and processes relating to variation in phenotype will be selected from:

An allele is an alternative form of a gene (one member of a pair) that is located at a specific position on a specific chromosome.

Organisms have two alleles for each trait. When the alleles of a pair are heterozygous, one is dominant and the other is recessive. The dominant allele is expressed and the recessive allele is masked.

The gene for seed shape in pea plants exists in two forms, one form or allele for round seed shape (R) and the other for wrinkled seed shape (r).

Organisms have two alleles for each trait. When the alleles of a pair are heterozygous, one is dominant and the other is recessive. The dominant allele is expressed and the recessive allele is masked. Using the previous example, round seed shape (R) is dominant and wrinkled seed shape (r) is recessive. Round: (RR) or (Rr), Wrinkled: (rr).

A mutation is a change in the base sequence of DNA caused by a mutagen. A mutagen is an agent, such as a chemical substance, UV light or radiation, that causes genetic mutation.

Mutation is a permanent / random changes in the DNA/ genetic material. Mutation must occur in gamete-producing cells to enter the gene pool of the population

A mutation is a permanent (unrepaired) change in an organisms DNA.

They introduce new alleles into a population. Most mutations are harmful.

Mutations are caused by mutagens.

Beneficial ones tend to occur more often in organisms with short generation times.

Many may be silent – not observed – and may only be selected for or against at a later date.

Neutral mutations make no change at all.

Beneficial mutation = A mutation that gives an organism a survival advantage.

Harmful mutation = A mutation that effects the survival of the organism.

Silent mutation = a mutation which has no observable effect on the organism.

(you are not required to provide the names of the stages of meiosis)

Sex cells have one set of chromosomes body cells have two. Click here to work through an animation

Online simulator for Mitosis and Meiosis Click here

Meiosis is a type of cell division that occurs in the testes (males) and ovaries (females). It produces four new cells (gametes) that are genetically different to each other, and to the parent cell. They contain half the number of chromosomes that are in the parent cell.Meiosis leads to genetic variation via two processes. When homologous pairs of chromosomes line up during meiosis, they do so randomly. This means it is completely random which combination of alleles end up in a particular gamete. This process is called independent assortment.The second way meiosis leads to genetic variation is via a process called crossing over. This occurs when homologous pairs of chromosomes line up at the cell equator and swap sections of genetic material, and therefore alleles. Because of crossing over, each gamete will contain different combinations of alleles.

Meiosis Explained

Mitosis explained (in more detail than you need at L1)

(in producing a new mix of alleles)

With sexual reproduction two individuals contribute genetic material with traits generally being determined by the two alleles for each gene. The process of meiosis which creates the gametes and recombination leads to an individual with a genetic make-up that differs from both parents. Over time that process allows the movements of alleles from one population to the next.

• Gametes are sex cells (sperm and egg) which are formed in the testes and ovaries. During gamete formation (meiosis), the homologous chromosomes are halved and the gamete will inherit one of each pair of chromosomes. Which chromosome is passed on is random due to the process of independent assortment.

• During fertilisation, the gametes combine and the resulting offspring will have two alleles – they may inherit two alleles the same, homozygous, and show that characteristic or they may inherit one of each allele, heterozygous in which case they will show the dominant allele in their phenotype. Genetic variation: variety within a population, eg different alleles possible for each gene. The advantage of variation to a population is that it may see some individuals survive if environment changes, in this case if drought occurs. Because of variation, not all individuals will be wiped out. Those with favourable alleles / traits / phenotypes will survive and be able to pass on genetic material to offspring and therefore survival of the species occurs.

• Possible disadvantages: need two parents that are able to reproduce, if conditions are stable could introduce variation, which may be counterproductive.

The patterns of inheritance involving simple monohybrid inheritance showing complete dominance

If you work through the student part of this animation series you will know everything you need to on monohybrid crosses.

Why the Human Genome Project Was Important

The Human Genome Project formed the first blueprint for a person and remains the largest collaborative biology project that humanity ever completed. Because the Project sequenced genomes of multiple organisms, scientist could compare them to uncover the functions of genes and to identify which genes are necessary for life.

Scientists took the information and techniques from the Project and used them to identify disease genes, devise tests for genetic diseases, and repair damaged genes to prevent problems before they occur. The information is used to predict how a patient will respond to a treatment based on a genetic profile. While the first map took years to complete, advances have led to faster sequencing, allowing scientists to study genetic variation in populations and more quickly determine what specific genes do.

The Project also included the development of an Ethical, Legal, and Social Implications (ELSI) program. ELSI became the largest bioethics program in the world and serves as a model for programs that deal with new technologies.

Watch the video: Απλοειδή και Διπλοειδή Κύτταρα-Ομόλογα Χρωμοσώματα (January 2022).