Key Definitions

Splicing increases transcript diversity via alternative splicing.

One of the most fascinating features of the nucleus is its ability to store approximately two meters of DNA within a space just a few microns in diameter. This incredible feat is achieved by compacting DNA into a structure called chromatin, which is a complex of DNA and proteins. The key proteins involved in chromatin formation are called histones.

These factors determine whether chromatin is open or closed, and thus whether genes are on or off.

The field of gene editing has undergone a revolution in the past decade, largely due to the discovery and development of a powerful tool known as CRISPR, short for Clustered Regularly Interspaced Short Palindromic Repeats. Originally discovered as part of a bacterial immune system, CRISPR is now one of the most precise, efficient, and affordable gene-editing technologies available. In this section, we'll explore the natural origins of CRISPR, how it works, and how scientists have adapted it for use in human cells to edit genes, cure diseases, and manipulate the immune system.

Methyl groups added to CpG islands (regions rich in cytosine and guanine).

Maintenance DNA methyltransferases (e.g., DNMT1), which recognize hemimethylated DNA and methylate the new strand.

To understand DNA replication, you must first understand the why behind the process. Every cell in your body contains a complete set of DNA,your genetic blueprint. When a cell divides, it must pass on an exact copy of that DNA to each of its two daughter cells. This ensures that both new cells can perform the same functions as the original and retain the organism's identity.

Environmental factors such as diet, stress, or toxins can alter gene packaging. In theory, these changes could be inherited. But biology has a safeguard: every generation, during the formation of sperm and egg, these epigenetic marks are typically wiped clean. This process resets the genetic slate and prevents acquired traits from being passed on.

Epigenetics is the study of how the same DNA can lead to different outcomes through changes in gene expression, without altering the underlying genetic code.

While gene editing with CRISPR-Cas9 introduces permanent changes by cutting DNA, epigenome editing modifies the epigenetic marks, such as DNA methylation or histone acetylation, that control whether a gene is turned "on" or "off."

DNA does not remain in a single condensation state throughout the cell's life cycle. Instead, it shifts between more open or closed configurations based on gene expression needs. These states are described as:

DNA itself can also be modified through methylation, particularly at CpG islands, regions rich in cytosine and guanine. Enzymes add methyl groups to the cytosine residues in these regions, which generally leads to transcriptional silencing. DNA methylation plays a crucial role in cellular differentiation, genomic imprinting, and X-chromosome inactivation.

DNA does not remain in a single condensation state throughout the cell's life cycle. Instead, it shifts between more open or closed configurations based on gene expression needs. These states are described as:

Added by: Histone acetyltransferases (HATs).

Can either activate or repress gene expression depending on the site and number of methyl groups.

In 1934, a determined Norwegian mother noticed something unusual about her two children. Both showed signs of developmental delay and intellectual disability, but what perplexed her most was the odd, musty odor of their urine. Believing the symptoms to be linked, she sought help. Eventually, she found Dr. Asbjørn Følling, a metabolic specialist and chemist who was fascinated by her story. Dr. Følling analyzed the children's urine and, after weeks of daily testing, identified phenylpyruvic acid,an abnormal compound. He soon linked it to a condition he called "imbecillitas phenylpyruvica," now known as phenylketonuria (PKU).

lncRNAs (>200 nt) can guide chromatin modifiers, act as scaffolds, or decoy transcription factors.

Of these, RNA Polymerase II is the main enzyme involved in mRNA synthesis.

These factors determine whether chromatin is open or closed, and thus whether genes are on or off.

At the heart of DNA condensation lies the nucleosome, the fundamental repeating unit of chromatin. A nucleosome consists of a segment of DNA wound twice around an octamer of histone proteins.

The nucleus is a specialized organelle that serves as the command center of eukaryotic cells. It is where the vast majority of a cell's genetic material is housed and protected, and it provides the environment in which gene expression and DNA replication take place. The structure of the nucleus is as crucial as its function; each component plays a specific role in supporting the life cycle of a cell.

In eukaryotic cells, pre-mRNA undergoes three major modifications before it becomes mature mRNA that can be translated into protein:

Found in centromeres, telomeres, and inactive X chromosomes

The supplied Burton and Greer review separates intergenerational effects from transgenerational effects by asking how many generations are affected and whether the relevant genetic material was directly exposed. In mammals, the review notes that environmental effects persisting for three or more generations through the female line, or two or more through the male line, are commonly described as transgenerational. For comparison across species, the authors treat a one-generation parent-to-offspring effect as intergenerational, effects lasting three or more generations as transgenerational, and two-generation observations on a case-by-case basis.