遗传 进化与生态学 8 - Genes

本期的内容是基因。本文集的这一部分是遗传、进化与生态学 Genetics, Evolution, and Ecology. 这门课理论上建议在阅读完文集的第一部分的内容之后再开始学习，但基础不足的朋友也可以尝试阅读喔~

这一部分的主要内容均来自 Prof. Angela J. Roles 的 BIOL 200 课程，因此本文集的这一部分均不会标记为原创。但由于文本来源不清晰，UP主还是一个字一个字码出来的文章，本文禁止非授权的转载，谢谢！

Lesson 8: Genes

[1] Gene Expression

Gene Expression: building cellular components

In the day to day life and function of a cell, protein is very important. In contrast, DNA makes up only a small fraction of the weight of a cell.

 Gene Expression

    ▸What: Process to make proteins starting from encoding gene;

    ▸When: Cells express genes in response to signals from their external environments;

    ▸How: DNA is transcribed into mRNA, mRNA is translated into protein;

    ▸Outcome: RNA and protein are produced (no change to amount of DNA in the cell).

[2] DNA Replication

    ▸DNA replication occurs during the synthesis (S) phase of the cell cycle;

    ▸In eukaryotes, the cell is in interphase during DNA replication (chromosomes not super-condensed);

    ▸All cells must undergo DNA replication in order to divide.

DNA Replication

    ▸What: Make a complete DNA copy of the dsDNA in the cell;

    ▸When: Before mitosis or meiosis in prep for cell division;

    ▸How: Before replication, genes are expressed to produce proteins necessary for replication;

    ▸Outcome: 2 complete copies of a cell’s DNA.

[3] The Central Dogma

    The central dogma of molecular biology is an explanation of the flow of genetic information within a biological system. It is often stated as "DNA makes RNA, and RNA makes protein."

    How do we get from genotype to phenotype?

    First, cells must produce proteins. That requires that genes are transcribed into RNA which is then processed and translated into protein;

    ▸Transcription = creation of RNA from DNA;

    ▸RNA splicing = processing the RNA to remove regions that will not be included in the final protein;

    ▸Export = preparing the processed RNA to be moved into the cytosol and translated into protein;

    ▸Translation = building of a protein according to the sequence of the messenger RNA.

▸In this eukaryotic cell, where does transcription occur? Where does translation occur?

▸In prokaryotes, transcription and translation occur in the cytosol (no nucleus present).

[4] Terms of Inheritance 

From genotype to phenotype

The Bar eye phenotype

    ▸The BarH1 gene encodes a transcription factor that plays a role in regulating proper eye development in the larvae.

    ▸The X(B) allele is a duplication of the BarH1 gene, which causes over-expression of the transcription factor.

[5] Gene Structure

Structure of a eukaryotic gene

Genes are particular lengths of the DNA sequence of a chromosome that encode the information to produce a protein (or functional RNA molecule).

    ▸Exon = nucleotide sequences (in DNA or RNA) that specify amino acids;

    ▸Intron = nucleotide sequences that do NOT specify amino acids and which interrupts the amino-acid encoding exons;

    ▸A single gene may include multiple exons and multiple introns.

 How big are genes?

    ▸On average, human protein-coding genes are about 3,000 nucleotides (or basepairs, bp). The shortest known is 500 nucleotides and the longest is 2.3 million.

    ▸The human genome contains 3.2 billion basepairs and 20–25 thousand genes.

Multiple genes on a chromosome

    ▸Only one strand of the double-helix codes for a given gene (but not all genes within a chromosome are on the same strand).

    ▸Sequences for different genes usually do NOT overlap each other.

    ▸Other sequences, including promoters, terminators, and transcription-factor binding sites, are important in regulating transcription of the gene.

    Note: Genes A and B are read left to right while Gene C is read right to left (they are on opposite strands). Positions of promoter and terminator are the clues.

[6] Example: BarH1 gene (3 exons, 2 introns)

Here’s a schematic of the actual BarH1 gene in Drosophila, from FlyBase:

 Gene span = from the first to the last basepair encoding an amino acid

Transcript = the part that is made into RNA

CDS = the part that is coding sequence (exons only)