This standard focuses on the process of Gene Expression, from how DNA is transcribed into RNA to how ribosomes form proteins through RNA translation.
Resources for this Standard:
- Overview (This article)
- HS-LS1-1 Quick Concept Quiz
- Teacher’s Checklist (Coming Soon!)
Here’s the Actual Standard:
Construct an explanation based on evidence for how the structure of DNA determines the structure of proteins, which carry out the essential functions of life through systems of specialized cells.
This standard focuses specifically on the processes of transcription and translation. During the first process, a double-stranded DNA molecule is opened with the help of a group of proteins broadly classified as “transcription factors”. These transcription factors open the DNA, then add complementary nucleotides to the DNA code using RNA elements. This creates an RNA molecule, known as the pre-messenger RNA (pre-mRNA).
During the second step, this RNA molecule undergoes a modification process called splicing. During splicing, this rough transcript is edited down to only include exons: the part of a gene that actually contains the genetic code to make a protein. The introns, which contain structural elements and sites for transcription factors to bind to, are removed from the pre-mRNA transcript.
Once this mRNA is completed, it exits the nucleus and makes its way to a ribosome. The ribosome assembles its two protein domains around the mRNA, and the process of translation begins. During translation, the ribosome matches transfer RNA (tRNA) molecules to each codon within the mRNA. Each codon consists of 3 nucleotides, each of which must form a complimentary bond with the nucleotides present on the tRNA molecule for a “match” to be recognized by the ribosome. Once this happens, the ribosome adds the amino acid attached to the tRNA to the growing protein chain.
Once all of the codons have been “read” and matched to a tRNA, the protein string is complete. However, before the protein can be used it must be folded into the proper shape. Some small proteins can fold automatically, directly in the cytosol. However, most proteins must undergo further processing in the endoplasmic reticulum or within a chaperone protein in order to be properly folded.
The final important piece of this standard relates to cellular differentiation. Each protein serves a function, and together many proteins create the overall function of a cell. Different cell types express different sets of proteins, which account for their different functions. The set of proteins expressed is controlled by the complex mechanisms of cellular differentiation and gene regulation, both of which can change which proteins are expressed in a cell. These processes are controlled by normal processes of growth and development, but they are also subject to environmental conditions like diet, exercise, toxins, and other external factors.
What to Avoid
The NGSS standard HS-LS1-1 also contains the following assessment boundary:
Assessment Boundary: Assessment does not include identification of specific cell or tissue types, whole-body systems, specific protein structures and functions, or the biochemistry of protein synthesis.
So, you don’t need to dive into the following topics in order to understand gene expression at the high school level.
Cell and Tissue Identification:
While the processes of cellular differentiation and gene expression should be covered, students do not need to be able to recognize different types of tissue under a microscope. Likewise, students do not need to recognize organ systems or how they interact (though this is covered in other NGSS standards).
Specific Protein Structures and Functions
At the high school level, students do not need to name the specific proteins involved in the processes of transcription, translation, or protein synthesis. However, students should be introduced to the complex cellular machinery involved in these processes. For example, students should learn about transcription factors and their role in transcription, though they do not need to know the names and exact function of every protein involved. Likewise, students should understand the process of protein folding, though an exploration of how proteins actually function is covered in other parts of the NGSS standards.
Biochemistry of Protein Synthesis
The assessment boundary on protein synthesis ends at biochemistry. So, discussions of peptide bonds, hydrogen bonding, and the biochemical nature of ribosomes do not need to be covered. In general, this standard is a general overview of gene expression and does not need to get into specific chemical reactions involved.