遗传 进化与生态学 6 - Information Transmission

本期的内容是（遗传）信息的传递。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 6: Information TransMission

[1] Overview

Information transMission from parent to offspring

    Division of genetic material:

    ▸Bacteria: binary fission

    ▸Eukaryotes: mitosis and meiosis

    ▸Archaea: variable—sometimes similar to bacteria or eukarya, sometimes unique

Schematic of Caulobacter bacterial cell cycle with asymmetric division:

The Life Cycle of a Cell:

 Important Events

▸Create a copy of the genomic DNA (the info).

▸Segregate (separate) the information into daughter cells.

▸Divide the cytoplasMic contents between daughter cells.

[2] Mitosis and cytokinesis

Slime mold, Dictyostelium discoideum

Plant cell cytokinesis

A cell plate grows from inside the cell out to the edges, separating the daughter cells.

[3] Binary Fission

- Binary fission in a bacterial life cycle: Bacillus licheniformis

- This bacterium lives in barbules of bird feathers and reproduces via binary fission. Under stress, it produces dormant spores.

▸What are the basic features of the life cycle?

    - Reproduction, ploidy

    - Habitat(s)

    - Trophic level

    - Adaptations

[4] Life Cycles

Consider life cycles: Culex, a mosquito

▸This eukaryote undergoes mitosis during growth and produces haploid gametes via meiosis;

▸Meiosis and mating with a mosquito of the opposite sex may increase genetic variation;

▸Survives stressful conditions by entering diapause from the egg, larval, or adult stages.

[5] Meiosis

Diploid nuclear division resulting in 4 haploid cells

    ▸Showing a genome with 1 chromosome (n=1 and 2n=2);

    ▸The B gene is present on this chromsome with two possible alleles: allele B and allele b;

    ▸Blue = chromosome copy inherited from parent 1;

    ▸Orange = chromosome copy inherited from Parent 2.

Meiosis in detail (Picture below)

[6] Crossing-Over

Synapsis - Physical pairing of homologous chromosomes.

▸ Recall: this is a 3-D structure;

▸Synapsis of homologous chromosomes (forming tetrads) happens because of similarity of DNA sequences;

▸Tetrads line up on the metaphase plate, leading to reduction from diploid to haploid nuclei.

Crossing-over occurs during synapsis; the physical connection enables the exchange between chromatids.

▸ Letters mark a particular place on the chromosome, like the address of a gene.

▸Centromeres are marked by the oval on the left of each chromatid;

▸Multiple crossing-over events may happen for each homologous pair.

[7] Meiosis I

Meiosis I (when all the good stuff happens)

[8] Mendel’s Laws

Mendel’s Laws on display

▸The Principle of Independent Assortment

    - All chromosomes inherited from one parent will not necessarily end up in the same daughter cell.

    - Non-homologous chromosomes sort independently of each other.

 ▸ Principle of Segregation

    - The 2 copies of 1 chromosome (1 set of genes) do not end up in the same daughter cell;

    - Each daughter cell contains 1 copy of each different chromosome, in the form of a replicated chromosome.

[9] Meiosis II

What’s happening in Meiosis II?

Questions to Think:

▸Why do you think biologists differentiate ploidy from chromosome copy number?

▸Compare binary fission to mitosis:

    - How are they different? Similar?

    - What are the relative advantages of each?

▸Compare mitosis to meiosis

    - How are they different? Similar?

    - What are the relative advantages of each?

▸When does each process occur in the life cycle of an organisM?

    - Does this suggest anything about the function or possible adaptive value of the process?

▸What might happen if there is an error during the process or some event fails to occur at all?

    - Are any of the resulting daughter cells likely to be viable?