Eukaryotes, whose chromosomes each consist of a linear DNA molecule, employ a different type of packing strategy to fit their DNA inside the nucleus.
At the most basic level, DNA is wrapped around proteins known as histones to form structures called nucleosomes. The histones are evolutionarily conserved proteins that are rich in basic amino acids and form an octamer. The DNA which is negatively charged because of the phosphate groups is wrapped tightly around the histone core. This nucleosome is linked to the next one with the help of a linker DNA.
This is further compacted into a 30 nm fiber, which is the diameter of the structure. At the metaphase stage the chromosomes are at their most compact, approximately nm in width, and are found in association with scaffold proteins. Eukaryotic chromosomes : These figures illustrate the compaction of the eukaryotic chromosome.
In interphase, eukaryotic chromosomes have two distinct regions that can be distinguished by staining. The tightly packaged region is known as heterochromatin, and the less dense region is known as euchromatin.
Heterochromatin usually contains genes that are not expressed, and is found in the regions of the centromere and telomeres. The euchromatin usually contains genes that are transcribed, with DNA packaged around nucleosomes but not further compacted. RNA is the nucleic acid that makes proteins from the code provided by DNA through the processes of transcription and translation.
DNA is the genetic material found in all living organisms and is found in the nucleus of eukaryotes and in the chloroplasts and mitochondria. In prokaryotes, the DNA is not enclosed in a membranous envelope. Each nucleotide is made up of three components: a nitrogenous base, a pentose five-carbon sugar called ribose, and a phosphate group.
RNA Structure : A nucleotide is made up of three components: a nitrogenous base, a pentose sugar, and one or more phosphate groups. Adenine A , guanine G , and cytosine C are present, but instead of thymine T , a pyrimidine called uracil U pairs with adenine. The DNA molecules never leave the nucleus but instead use an intermediary to communicate with the rest of the cell. This is called transcription. The mRNA then carries the code out of the nucleus to organelles called ribosomes for the assembly of proteins.
Once the mRNA has reached the ribosomes, they do not read the instructions directly. It then reads the sequence in sets of three bases called codons. Each possible three letter arrangement of A,C,U,G e. The ribosome acts like a giant clamp, holding all of the players in position, and facilitating both the pairing of bases between the messenger and transfer RNAs, and the chemical bonding between the amino acids.
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Figure 1. Figure Detail. Figure 2. Degeneracy of the Amino Acid Code. Figure 3: The amino acids specified by each mRNA codon. Multiple codons can code for the same amino acid. The codons are written 5' to 3', as they appear in the mRNA. References and Recommended Reading Crick, F.
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