Each cell normally contains 46 chromosomes, arranged in 23 pairs—one set from each parent.

 

[What are chromatids?]

Key features

In contrast

After DNA replication (during S phase), each chromosome becomes a duplicated chromosome, which consists of:

Simple analogy

Clinical/board relevance

 

The short arm region is called the p arm and the long arm region is called the q arm. The end region of a chromosome is called a telomere. Telomeres consist of repeating non-coding DNA sequences that get shorter as a cell divides.

A duplicated chromosome is comprised of two identical chromosomes called sister chromatids that are connected at the centromere region.

The structure of chromosomes and chromatin varies through the cell cycle.

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Function

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Deoxyribonucleic acid (DNA) is the nucleic acid involved in the genetic code. DNA sequences contain the genetic instructions used in the development and functioning of all known living organisms except RNA viruses.

It is the sequence of these four nucleobases along the backbone that encodes genetic information used in the development and functioning of all known living organisms except RNA viruses. The DNA sequences have structural purposes and are also involved in regulating the use of genetic information. This information is read using the genetic code, which specifies the sequence of the amino acids within proteins. The code is read by copying stretches of DNA into the related nucleic acid RNA in a process called transcription.

Heredity

DNA packs in all the genetic information and passes it on to the next generation. DNA holds the instructions for an organism's or each cell’s development and reproduction and ultimately death.

This is accomplished throguh the process of DNA replication

In this process the DNA strands, that are tightly wound with each other, unwind and literally unzip to leave several bases without their partners on the other strand and remain along the backbone of the molecule.

The bases are very specific about which base they will attach to and the adenine only pairs with thymine and guanine will only pair with cytosine. Unpaired bases come and attach to these free bases and a new strand is formed that is complementary to the original sequence.

The end result is a strand that is a perfect match to the original one prior to it unzipping. This result in two new pairs of strands and two coiled DNA. Each of the new DNA contains one strand from the mother pair and a new one.

This is used in in reproduction, maintenance and growth of cells, tissues and body systems.

Making Protein

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It is the sequence of these four nucleobases along the backbone that encodes genetic information used in the development and functioning of all known living organisms except RNA viruses. The DNA sequences have structural purposes and are also involved in regulating the use of genetic information. This information is read using the genetic code, which specifies the sequence of the amino acids within proteins. The code is read by copying stretches of DNA into the related nucleic acid RNA in a process called transcription.

Although DNA contains the genetic blueprint of life, it requires the assistance of ribonucleic acid (RNA) to be functional. After DNA is converted into strands of RNA, the messenger RNA (mRNA) is sent to the ribosome to direct the synthesis of proteins.

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The code is read by copying stretches of DNA into the related nucleic acid RNA in a process called transcription which specifies the sequence of the amino acids within proteins.

Within cells DNA is organized into long structures called chromosomes. During cell division these chromosomes are duplicated in the process of DNA replication, providing each cell its own complete set of chromosomes.

Eukaryotic organisms (animals, plants, fungi, and protists) store most of their DNA inside the cell nucleus and some of their DNA inorganelles, such as mitochondria or chloroplasts.[1] In contrast, prokaryotes (bacteria and archaea) store their DNA only in the cytoplasm. Within the chromosomes, chromatin proteins such as histones compact and organize DNA. These compact structures guide the interactions between DNA and other proteins, helping control which parts of the DNA are transcribed.

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For more information on DNA read the following:

1. http://en.wikipedia.org/wiki/DNA

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Animated Graphic DNA Structure: http://www.johnkyrk.com/DNAanatomy.swf ( John Kyrk Master of Biology, Harvard and Animator.)

Conccccccccccccctent 3

a. Transcription

The first step that occurs is a process known as transcription. DNA is read by the (who does this) messengers that break it open into single stranded polynucleotide chains and is copied into RNA. RNA acts as a messenger to carry the information to other parts of the cell.

RNA forms opposite bases from that present on the DNA. For example, G on the DNA forms C on the RNA strand.

Each of the bases gets together in threes and these form particular amino acids. There are 20 such amino acids. These are also known as the building blocks of proteins.

The amino acids first form a long chain called the polypeptide chain. This polypeptide chain undergoes conformational and structural changes and folds and refolds over itself to form the final complex structure of the protein.

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b. Translation

The next step is translation. In this step the cell organelles called ribosomes come into play. These ribosomes act as translators by translating the messenger's code into the proper protein format or a chain of amino acids that form the building blocks of the protein. Proteins are created by ribosomes translating mRNA into polypeptide chains. Each amino acid is formed by combining three bases on the RNA.

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c. Posttranslational modification (PTM)

This is the third step before sending it to the required areas in the body. These polypeptide chains undergo PTM, (such as folding, cutting and structuring), before becoming the mature protein product.

Posttranslational modification of insulin. At the top, the ribosome translates a mRNA sequence into a protein, insulin, and passes the protein through the endoplasmic reticulum, where it is cut, folded and held in shape by disulfide (-S-S-) bonds. Then the protein passes through the golgi apparatus, where it is packaged into a vesicle. In the vesicle, more parts are cut off, and it turns into mature insulin.

A protein (also called a polypeptide) is a chain of amino acids. During protein synthesis, 20 different amino acids can be incorporated to become a protein. After translation, the posttranslational modification of amino acids extends the range of functions of the protein by attaching it to other biochemical functional groups (such as acetate, phosphate, various lipids and carbohydrates), changing the chemical nature of an amino acid (e.g. citrullination), or making structural changes (e.g. formation of disulfide bridges).

Also, enzymes may remove amino acids from the amino end of the protein, or cut the peptide chain in the middle. For instance, the peptide hormone insulin is cut twice after disulfide bonds are formed, and a propeptide is removed from the middle of the chain; the resulting protein consists of two polypeptide chains connected by disulfide bonds. Also, most nascent polypeptides start with the amino acid methionine because the "start" codon on mRNA also codes for this amino acid. This amino acid is usually taken off during post-translational modification.

Other modifications, like phosphorylation, are part of common mechanisms for controlling the behavior of a protein, for instance activating or inactivating an enzyme.

Post-translational modification of proteins is detected by mass spectrometry or Eastern blotting.

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Human cells normally contain 23 pairs of chromosomes, for a total of 46 chromosomes in each cell.

Humans have 23 pairs of chromosomes – a total of 46 chromosomes. Twenty-two of these pairs, called autosomes, look the same in both males and females. The 23rd pair is called the sex chromosomes and differs between males and females. Females have two copies of the X chromosome or XX, while males have one X and one Y chromosome.

Both parents have reproductive cells – sperms in fathers and ovum or eggs in mothers. These sperms and eggs contain half the number of chromosomes – 23 each. When the egg and the sperm fertilizes, this gives rise to a cell that has the complete set. Thus a person inherits half of his or her genes from each of the parents.

Human cells normally contain 23 pairs of chromosomes, for a total of 46 chromosomes in each cell.

Humans have 23 pairs of chromosomes – a total of 46 chromosomes. Twenty-two of these pairs, called autosomes, look the same in both males and females. The 23rd pair is called the sex chromosomes and differs between males and females. Females have two copies of the X chromosome or XX, while males have one X and one Y chromosome.

Both parents have reproductive cells – sperms in fathers and ovum or eggs in mothers. These sperms and eggs contain half the number of chromosomes – 23 each. When the egg and the sperm fertilizes, this gives rise to a cell that has the complete set. Thus a person inherits half of his or her genes from each of the parents.

 

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Although females possess two copies of the X chromosome, not all genes on both chromosomes are expressed due to the process of X chromosome inactivation, XCI (see Chapter 22).

FIGURE 35–1

Conventional numbering system of G bands in human chromosomes. Karyotypes are ways of organizing and presenting the chromosomal makeup of an individual. (Reproduced with permission from Brooker RJ: Genetics: Analysis & Principles, 7th ed. New York, NY: McGraw Hill, 2021.)

In addition to being of different sizes the chromosomes are divided into two distinct arms on either side of the centromeric domains of each chromosome. The centromere of each chromosome is a highly condensed region of exclusively repetitive DNA (termed satellite DNA) that is the site at which the microtubules of the spindle attach, via the kinetochore, during cell division (see Chapter 22). The longer of the two chromosome arms is referred to as the q arm and the shorter as the p arm.

In some chromosomes, the size of the p and q arms is very similar such that the centromere is very nearly in the middle of the chromosome. These latter chromosomes are termed metacentric chromosomes (1, 3, 16, 19, 20). Chromosomes with longer q arms are referred to as submetacentric chromosomes (2, 4-12, 17, 18, and X). A small subset of chromosomes (13, 14, 15, 21, 22, and Y) have nearly nonexistent p arms and these chromosomes are termed the acrocentric chromosomes.

The contribution of DNA from both the human male and female constitutes the inherited genetic material that defines the characteristics of their offspring. These characteristics are determined primarily by the composition of protein-coding genes in the inherited chromosomes. Of the vast amount of DNA in the human genome only around 1% is devoted to protein-coding genes. Indeed, the current estimate is that there are approximately 21,000 protein-coding genes in the human genome. However, due to alternative promoter usage and alternative mRNA splicing (see Chapter 23) a vastly larger total number of distinct proteins is derived from the human genome.

 

 

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