Which of the following molecules is the subunit of dna that links together to form strands of dna?

DNA (deoxyribonucleic acid) is a molecule that stores genetic information within living organisms and provides the instructions that cells need to function and reproduce. DNA is made of smaller subunits called nucleotides. Each nucleotide consists of three components: a five-carbon sugar called deoxyribose, a phosphate group, and a nitrogenous base. The four types of DNA nucleotides are distinguished based on which nitrogenous base they contain—adenine (A), guanine (G), thymine (T), or cytosine (C).

What Is a Strand of DNA Made of and How Is It Made?

A strand of DNA is made up of nucleotides that form a chain, creating a long, linear molecule. To form an individual strand, the sugar molecule of one nucleotide forms a covalent bond with the phosphate group of the adjacent nucleotide, forming a strong sugar-phosphate backbone.

What Information Is Contained in DNA?

DNA contains all of the genetic information necessary for an organism to develop, function, and reproduce. DNA encodes this information as specific sequences of nucleotide bases.

One of the most important types of information contained in DNA is the instructions for how to build various proteins, as proteins do most of the work in cells. To follow these instructions, a cell must first copy a gene into a form of RNA known as precursor messenger RNA (pre-mRNA). This process is called transcription.

After being processed, the RNA (now called mRNA or mature mRNA) is ready to be translated into a protein that can carry out the instructions in the gene.

Genes that encode proteins only account for around two percent of our total DNA sequence. The rest of an organism's DNA was once seen as 'junk DNA', but we now know that it can play other roles, including regulatory ones like influencing gene transcription.

How Is DNA Formed?

Copies of DNA molecules are formed through a process known as DNA replication. The first step in this process involves breaking the hydrogen bonds that connect the two strands of the DNA double helix at specific places called origins of replication, so that the double helix splits in two within the replication bubble around the origin of replication.

An enzyme called DNA polymerase then uses complementary base pairing (where A binds to T and G binds to C) to make a copy of each strand. For example, if there is an adenine (A) in the DNA strand, the DNA polymerase will add a thymine (T) to the newly forming strand—and so on, resulting in a complementary strand of DNA. Thus two copies of the DNA molecule are created from the original, in what is called semiconservative replication.

On occasion, DNA polymerase makes a mistake during replication, accidentally matching the wrong bases together. When this occurs, the polymerase pauses while other enzymes work to repair the error, and resumes its replication once the DNA mistake has been fixed.

While these error-checking mechanisms provide a high degree of accuracy, the polymerase still make mistakes: everyone is born with approximately one difference per every 100 million bases compared to their parents. These uncorrected mistakes become mutations, which can occasionally lead to disease. But they also help to explain why every human in unique.

The genetic information stored in DNA is a living archive of instructions that cells use to accomplish the functions of life. Inside each cell, catalysts seek out the appropriate information from this archive and use it to build new proteins — proteins that make up the structures of the cell, run the biochemical reactions in the cell, and are sometimes manufactured for export. Although all of the cells that make up a multicellular organism contain identical genetic information, functionally different cells within the organism use different sets of catalysts to express only specific portions of these instructions to accomplish the functions of life.

How Is Genetic Information Passed on in Dividing Cells?

When a cell divides, it creates one copy of its genetic information — in the form of DNA molecules — for each of the two resulting daughter cells. The accuracy of these copies determines the health and inherited features of the nascent cells, so it is essential that the process of DNA replication be as accurate as possible (Figure 1).

One factor that helps ensure precise replication is the double-helical structure of DNA itself. In particular, the two strands of the DNA double helix are made up of combinations of molecules called nucleotides. DNA is constructed from just four different nucleotides — adenine (A), thymine (T), cytosine (C), and guanine (G) — each of which is named for the nitrogenous base it contains. Moreover, the nucleotides that form one strand of the DNA double helix always bond with the nucleotides in the other strand according to a pattern known as complementary base-pairing — specifically, A always pairs with T, and C always pairs with G (Figure 2). Thus, during cell division, the paired strands unravel and each strand serves as the template for synthesis of a new complementary strand.

What Are the Initial Steps in Accessing Genetic Information?

RNA molecules differ from DNA molecules in several important ways: They are single stranded rather than double stranded; their sugar component is a ribose rather than a deoxyribose; and they include uracil (U) nucleotides rather than thymine (T) nucleotides (Figure 4). Also, because they are single strands, RNA molecules don't form helices; rather, they fold into complex structures that are stabilized by internal complementary base-pairing.

mRNA is the most variable class of RNA, and there are literally thousands of different mRNA molecules present in a cell at any given time. Some mRNA molecules are abundant, numbering in the hundreds or thousands, as is often true of transcripts encoding structural proteins. Other mRNAs are quite rare, with perhaps only a single copy present, as is sometimes the case for transcripts that encode signaling proteins. mRNAs also vary in how long-lived they are. In eukaryotes, transcripts for structural proteins may remain intact for over ten hours, whereas transcripts for signaling proteins may be degraded in less than ten minutes.

Cells can be characterized by the spectrum of mRNA molecules present within them; this spectrum is called the transcriptome. Whereas each cell in a multicellular organism carries the same DNA or genome, its transcriptome varies widely according to cell type and function. For instance, the insulin-producing cells of the pancreas contain transcripts for insulin, but bone cells do not. Even though bone cells carry the gene for insulin, this gene is not transcribed. Therefore, the transcriptome functions as a kind of catalog of all of the genes that are being expressed in a cell at a particular point in time.

What Is the Function of Ribosomes?

Ribosomes are complexes of rRNA molecules and proteins, and they can be observed in electron micrographs of cells. Sometimes, ribosomes are visible as clusters, called polyribosomes. In eukaryotes (but not in prokaryotes), some of the ribosomes are attached to internal membranes, where they synthesize the proteins that will later reside in those membranes, or are destined for secretion (Figure 6). Although only a few rRNA molecules are present in each ribosome, these molecules make up about half of the ribosomal mass. The remaining mass consists of a number of proteins — nearly 60 in prokaryotic cells and over 80 in eukaryotic cells.

Within the ribosome, the rRNA molecules direct the catalytic steps of protein synthesis — the stitching together of amino acids to make a protein molecule. In fact, rRNA is sometimes called a ribozyme or catalytic RNA to reflect this function.

Eukaryotic and prokaryotic ribosomes are different from each other as a result of divergent evolution. These differences are exploited by antibiotics, which are designed to inhibit the prokaryotic ribosomes of infectious bacteria without affecting eukaryotic ribosomes, thereby not interfering with the cells of the sick host.

How Does the Whole Process Result in New Proteins?

After the transcription of DNA to mRNA is complete, translation — or the reading of these mRNAs to make proteins — begins. Recall that mRNA molecules are single stranded, and the order of their bases — A, U, C, and G — is complementary to that in specific portions of the cell's DNA. Each mRNA dictates the order in which amino acids should be added to a growing protein as it is synthesized. In fact, every amino acid is represented by a three-nucleotide sequence or codon along the mRNA molecule. For example, AGC is the mRNA codon for the amino acid serine, and UAA is a signal to stop translating a protein — also called the stop codon (Figure 7).

Molecules of tRNA are responsible for matching amino acids with the appropriate codons in mRNA. Each tRNA molecule has two distinct ends, one of which binds to a specific amino acid, and the other which binds to the corresponding mRNA codon. During translation, these tRNAs carry amino acids to the ribosome and join with their complementary codons. Then, the assembled amino acids are joined together as the ribosome, with its resident rRNAs, moves along the mRNA molecule in a ratchet-like motion. The resulting protein chains can be hundreds of amino acids in length, and synthesizing these molecules requires a huge amount of chemical energy (Figure 8).

In prokaryotic cells, transcription (DNA to mRNA) and translation (mRNA to protein) are so closely linked that translation usually begins before transcription is complete. In eukaryotic cells, however, the two processes are separated in both space and time: mRNAs are synthesized in the nucleus, and proteins are later made in the cytoplasm.

Conclusion

Cellular DNA contains instructions for building the various proteins the cell needs to survive. In order for a cell to manufacture these proteins, specific genes within its DNA must first be transcribed into molecules of mRNA; then, these transcripts must be translated into chains of amino acids, which later fold into fully functional proteins. Although all of the cells in a multicellular organism contain the same set of genetic information, the transcriptomes of different cells vary depending on the cells' structure and function in the organism.