Chapter 13 Meiosis and The Behavior of Chromosomes

Inheritance/Heredity n When traits are passed down from one generation to the next, we say they are inherited. n The offspring differ from one generation to the next and we use the term variation to describe these differences.

Chromosomes n Chromosomes carry genes from one organism to the next within a species. n They are the vehicles of heredity. n Minor differences in the sequences of base pairs on these chromosomes is what contributes to variation.

Prokaryotic Chromosomes n Bacteria have one circular chromosome that is often associated with one or more plasmids. n Plasmids are pieces of genetic material that often carry genes that help the organism.

Eukaryotic Chromosomes n Eukaryotic chromosomes are linear molecules of DNA associated with histone proteins. n Different chromosomes carry different genes.

Genes n Genes are regions of our genome that program the specific traits we see (phenotype) such as eye color, height, or many other factors.

Chromosomal Length n Through a variety of techniques, scientists learned a lot about chromosomes, but questions remained as to how long they were. n To determine the length of the chromosome, scientists had to employ a variety of different techniques. 7

John Cairns did a lot of elegant experiments using radioactive tracers to show DNA was replicated. n His technique was also used to determine the length of DNA found inside a cell. https: //www. orau. org/ptp/collection/Radiology/Cassettes. htm

Cairns’ Technique used 3 H-thymidine to radioactively label the newly made strands of DNA in E. coli cells. n Next he gently lysed the cells and placed them onto an EM grid and exposed them to X-ray film for varying lengths of time. n This showed the circular nature of the bacterial chromosomes he was working with. n He

Cairns’ Technique Results http: //schaechter. asmblog. org/schaechter/2015/01/pictures-considered-23 -what-grains-tell. html http: //schaechter. asmblog. org/schaechter/2013/03/pictures-considered-the-e-coli-chromosome-caught-in-the-act-of-replicating. html

Cairns’ Technique n After finding success using this method, Cairns and others were able to modify the technique and determine the length of chromosomes in eukaryotic cells. Huberman and Riggs, 1968 J. Mol. Biol. 32: 327 -341. Autoradiography of Chromosomal DNA Fibers from Chinese Hamster Cells, Huberman and Riggs, 1966.

Homologous Chromosomes n Homologous chromosomes are chromosomes of the same number. n Each homologous chromosome contains genes that control the same inherited characteristic. – For example, if the gene that codes for height is found on one chromosome, it is found at the exact same locus on the homologous chromosome.

The Locus n The locus is the region of the chromosome where a particular gene is located.

Homologous Chromosomes n Diploid organisms have pairs of homologous chromosomes within their nuclei. n During the process of meiosis, the chromosome number is halved.

Homologous Chromosomes--Sort Of… n The only two chromosomes that don’t really fit the idea of being homologous are the sex chromosomes, the X and the Y. n The two sets of genes on these chromosomes are more different than they are alike. n They determine the sex of the individual: XX-female; XY-male

Chromosome Number. Autosomes n When discussing chromosome number, we use an “n” to represent this idea. n Any cell having both copies of chromosomes from the parent are said to be diploid which is 2 n. n In humans, 2 n=46.

Chromosome Number-Sex Chromosomes n The gametes of sexually reproducing organisms contain a single chromosome set and are said to be haploid. n n=23 (22 autosomes + 1 sex c-some). n Eggs and sperm are haploid.

Fertilization n When 2 haploid cells fuse together, a diploid zygote is formed. n The zygote now contains a mix of genes from the two parents. n It will divide mitotically and give rise to a mature organism.

So, how does this happen? n The new individuals now have traits of both parents. n Recall, the chromosome that is inherited is tightly coiled and contains a few hundred to a few thousand genes.

Modes of Reproduction n When discussing heredity, there a couple of modes of reproduction that need to be discussed. n Sexual and asexual.

Sexual Reproduction n 2 parents “mate” and give rise to offspring that have a variety of combinations of genes which are inherited by the new offspring. n They are not genetically identical to the parents.

Asexual Reproduction n. A single parent gives rise to the offspring. n In the process, all of the parent’s genes are passed on to the offspring and a clone is formed. – A hydra and yeast are examples of asexually reproducing organisms. – They reproduce by budding off clones of themselves.

The Sexual Lifecycle--Humans n Somatic cells are the cells other than the sperm and egg. n Each one contains 46 chromosomes (23 pairs, 1 pair from each parent). n Each parent contributes 22 autosomes and 1 sex c-some.

Genetics n When people study genetics, they are studying heredity and inherited variations. n For example, when we say we have our parent’s nose or eyes, we are actually saying that we have their segments of DNA (genes).

So, how does this happen? n How do the genes get passed on from one generation to the next? n The exact replication and random mixing of our genes/DNA in our gametes produces copies that can be passed from one generation to the next. n This is the topic of the next presentation.