Biology unit 2 notes

Cell membrane and membrane-bound organelles Subcellular components All cells, whether prokaryotic or eukaryotichave a membrane that envelops the cell, regulates what moves in and out selectively permeableand maintains the electric potential of the cell.

Biology unit 2 notes

Published May 22, By Dr. This genome encodes approximately proteins. In comparison, the genome within a human cell contains 23 pairs of chromosomes ranging in size from 50 to Mb. Approximately 40, genes are present within the Mb of human DNA. Consequently, more-elaborate mechanisms are required to achieve specificity.

Liver and pancreatic cells, for example, differ dramatically in the genes that are highly expressed. Instead, genes that encode proteins for steps within a given pathway are often spread widely across the genome.

Mechanism of regulation of gene expression in Eukaryotes 1 Chromatin Remodeling Chromatin structure provides an important level of control of gene transcription. Large regions of chromatin are transcriptionally inactive while others are either active or potentially active. With few exceptions, each cell contains the same complement of genes antibody-producing cells are a notable exception.

The development of specialized organs, tissues, and cells and their function in the intact organism depend upon the differential expression of genes.

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Some of this differential expression is achieved by having different regions of chromatin available for transcription in cells from various tissues. Formation and disruption of nucleosome structure The presence of nucleosomes and of complexes of histones and DNA certainly provides a barrier against the ready association of transcription factors with specific DNA regions.

The dynamics of the formation and disruption of nucleosome structure are Biology unit 2 notes an important part of eukaryotic gene regulation and the processes involved are as follows- i Histone acetylation and deacetylation is an important determinant of gene activity.

Acetylation is known to occur on lysine residues in the amino terminal tails of histone molecules Figure This modification reduces the positive charge of these tails and decreases the binding affinity of histone for the negatively charged DNA.

Accordingly, the acetylation of histones could result in disruption of nucleosomal structure and allow readier access of transcription factors to cognate regulatory DNA elements. Different proteins with specific acetylase and deacetylase activities are associated with various components of the transcription apparatus.

The positive charge is removed after acteylation. Thus, histone acetylation can activate transcription through a combination of three mechanisms: Methylation of deoxycytidine residues Figure-3 in DNA may effect gross changes in chromatin so as to preclude its active transcription.

Acute demethylation of deoxycytidine residues in a specific region of the tyrosine aminotransferase gene—in response to glucocorticoid hormones—has been associated with an increased rate of transcription of the gene.

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However, it is not possible to generalize that methylated DNA is transcriptionally inactive, that all inactive chromatin is methylated, or that active DNA is not methylated.

Such protein-protein interactions play a dominant role in eukaryotic gene regulation. In contrast with those of prokaryotic transcription, few eukaryotic transcription factors have any effect on transcription on their own.

Instead, each factor recruits other proteins to build up large complexes that interact with the transcriptional machinery to activate or repress transcription. A major advantage of this mode of regulation is that a given regulatory protein can have different effects, depending on what other proteins are present in the same cell.

This phenomenon, called combinatorial control, is crucial to multicellular organisms that have many different cell types. The binding of specific transcription factors to certain DNA elements may result in disruption of nucleosomal structure. Many eukaryotic genes have multiple protein-binding DNA elements.

The serial binding of transcription factors to these elements may either directly disrupt the structure of the nucleosome or prevent its re-formation.

Biology unit 2 notes

These reactions result in chromatin-level structural changes that in the end increase DNA accessibility to other factors and the transcription machinery. Enhancers function by serving as binding sites for specific regulatory proteins.

An enhancer is effective only in the specific cell types in which appropriate regulatory proteins are expressed. In many cases, these DNA-binding proteins influence transcription initiation by perturbing the local chromatin structure to expose a gene or its regulatory sites rather than by direct interactions with RNA polymerase.

Enhancers are promiscuous; they can stimulate any promoter in the vicinity and may act on more than one promoter. The elements that decrease or repress the expression of specific genes have also been identified. Silencers are control regions of DNA that, like enhancers, may be located thousands of base pairs away from the gene they control.

However, when transcription factors bind to them, expression of the gene they control is repressed.

Biology unit 2 notes

Tissue-specific gene expression is mediated by enhancers or enhancer-like elements. Many genes are now recognized to harbor enhancer or activator elements in various locations relative to their coding regions.

In addition to being able to enhance gene transcription, some of these enhancer elements clearly possess the ability to do so in a tissue-specific manner. Thus, the enhancer element associated with the immunoglobulin genes between the J and C regions enhances the expression of those genes preferentially in lymphoid cells.Honors Biology.

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Lewis Thomas was a physician, poet, etymologist, essayist, administrator, educator, policy advisor, and researcher. A graduate of Princeton University and Harvard Medical School, he was the dean of Yale Medical School and New York University School of Medicine, and the president of Memorial Sloan-Kettering Institute.

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