Are there always going to be an equal number of cytosine and guanine nucleotides adenine and thymine nucleotides in A DNA molecule Why?

DNA is a macro molecule that consists of many subunits connected together. The subunits are called nucleotides. Each nucleotide has three parts: a sugar, a phosphate, and a base. The sugar and phosphate molecules are linked together in two long chains. The bases are linked to the sugar-phosphates. Bases of one strand are bound to those of the other strand by hydrogen bonds making what is called a base pair. The structure of DNA is a double helix which allows it to perform the functions of replication and information storage. Base pairs look similar to rungs on a ladder. In fact, the DNA structure could be described as a long ladder twisted into a spiral.

DNA has a double helix structurecomposed of nucleotides. (Image by P. Hain)

There are only 4 nucleotides in DNA, Adenine (A), Guanine (G), Thymine (T), and Cystosine (C). The chemical structures of Thymine and Cytosine are smaller, while those of Adenine and Guanine are larger. Size and structure of the specific nucleotides cause Adenine and Thymine to always pair together while Cytosine and Guanine always pair together. Therefore the two strands of DNA are considered complimentary.

The four nucleotide structures. (Images by P. Hain and University of Nebraska - Lincoln)

Adenine-thymine and cystine-guanine interactions (Images by University of Nebraska - Lincoln)

The nucleotides are like letters in the ’genetic language’. Just as we use letters to make words with meaning, the order of the nucleotides on a DNA strand codes information. They make ’words’ that tell the cell how to make each protein. Furthermore, the genetic language is a universal language. Every living organism uses the same nucleotide combinations to code for its genetic information. This characteristic is important in genetic engineering. It allows the transfer of genetic information from one species to another while maintaining its meaning.

*This animation has no audio.*

A base pair consists of two complementary DNA nucleotide bases that pair together to form a “rung of the DNA ladder.” DNA is made of two linked strands that wind around each other to resemble a twisted ladder — a shape known as a double helix. Each strand has a backbone made of alternating sugar (deoxyribose) and phosphate groups. Attached to each sugar is one of four bases: adenine (A), cytosine (C), guanine (G) [GWA-NeeN] or thymine (T). The two strands are held together by hydrogen bonds between pairs of bases: adenine pairs with thymine, and cytosine pairs with guanine. 

One copy of the human genome consists of approximately 3 billion base pairs of DNA, which are distributed across 23 chromosomes. Human chromosomes range in size from about 50 million to 300 million base pairs. Because the bases exist as pairs, and the identity of one of the bases in the pair determines the other member of the pair, scientists do not have to report both bases of the pair — which is why DNA sequence is typically represented as single strings of letters. DNA sequencing involves determining the exact order of the base pairs across a DNA segment of interest or across an entire genome. A signature goal of the Human Genome Project was to generate the first high-quality sequence of the human. The effort was successful in generating a such a sequence for over 90% of the human genome, but it took nearly two more decades to sequence the remaining bits of the human genome — which were heavily enriched for highly repetitive and difficult-to-sequence stretches of DNA.

mariandalogdog97 mariandalogdog97

Answer:

It is always an equal number of Guanina and cytosine

Explanation:

Because the number of Guanine present should always be equal to the number of Cytosine present otherwise DNA molecules cannot pair with each other or they can't form the double strand of DNA.