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Genetics/Genetic modification.



I am reading a book on GMOs, more specific GMOs in animals/parasites.
One of the sentences in the book is a bit difficult for me to understand.
It says: "the unicellular parasite is haploid for most of its life cycle, a potential problem if transformation is mediated by homologous recombination."

What do they mean? I think they mean that the "mechanisms" that provide homologous recombinations are not active during the haploid live stage because when its haploid no cell division takes place? (accepting it does not divide asekually like bacteria).
Or is it more then that?

Also: am I right to presume that an unicellular parasite will not "renew" its DNA (like for example humans do for cell division or bacteria) unless it is going to aseksually divide/reproduce?
And also meaning: only when it will start to reproduce seksually it will start to multiply its DNA.

Thanks in advance.

Hi Luc!

Homologous recombination is an event where two sister chromosomes (or genomes) exchange genetic material between the two copies for editing and error checking. It's problematic for a haploid organism to utilize homologous recombination to transform itself because when an organism is haploid, by definition there is only one copy of the DNA! There's nothing for the DNA to exchange with. Homologous recombination is only active during the diploid (two copies) part of the parasite's life cycle.

It sounds like this particular parasite goes through a couple life phases, one where it is haploid, possibly reproducing asexually, and another where it is diploid and can reproduce sexually. Homologous recombination is also how organisms reshuffle their DNA during sexual reproduction to try and generate new gene combinations in their offspring for maximal survival of the species.

So if a unicellular parasite is about to divide, it will copy its genome and then split in half, with each daughter cell getting one-half of the DNA. (mitosis)

If a diploid parasite is going to reproduce sexually, it will copy the 2N genome to 4N, assort a single N into one of four daughter cells, and then combine that sex cell with one of another sex cell from a different parasite. (meiosis)

I'll include the wiki articles for mitosis and meiosis. :) They will probably help clear things up immensely.


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Trista Robichaud, PhD


No homework questions, especially ones copied and pasted from textbooks. I will answer questions about principles or give hints, but I do not do other's homework. I'm comfortable answering basic biochemistry, chemistry, genetics, and biology questions up to and including an undergraduate level of understanding. This includes molecular biology, protein purification, and genetics. My training/inclination is primarily in structural biology, or how the shapes of things affect their function. Other interests include protein design, protein engineering, enzyme kinetics, and metabolic diseases such as cancer, atherosclerosis, and diabetes. Regrettably, I cannot diagnose any disease. I can tell you how we currently understand the basic science behind a disease state, but I cannot recommend treatment in any way. Please direct such questions to your medical professional.


I hold a PhD in Biomedical Science from the University of Massachusetts Medical School in Worcester. I specialize in Biochemistry, with a focus on protein chemistry. My thesis work involved the structure and functions of the human glucose transporter 1. (hGLUT1) Currently I am a postdoc working in peptide (mini-protein) design and enzymology at the University of Texas Health Science Center in San Antonio, Texas. I am in Bjorn Steffensen's lab, researching inhibitors of gelatinase A, a matrix metalloproteinase. I have also been answering Chemistry/Biochemistry questions on this site since summer 2010.

2001 American Association for the Advancement of Science, 2007 American Chemical Society 2007 Protein Society 2011 UTHSCSA Women’s Faculty Association

Publications Levine KB, Robichaud TK, Hamill S, Sultzman LA, Carruthers A. Properties of the human erythrocyte glucose transport protein are determined by cellular context. Biochemistry 44(15):5606-16, 2005. (PMID 15823019) Robichaud TK, Appleyard AN, Herbert RB, Henderson PJ, Carruthers A “Determinants of ligand binding affinity and cooperativity at the GLUT1 endofacial site” Biochemistry 50(15):3137-48, 2011. (PMID 21384913) Xu X, Mikhailova M, Chen Z, Pal S, Robichaud TK, Lafer EM, Baber S, Steffensen B. “Peptide from the C-terminal domain of tissue inhibitor of matrix metalloproteinases-2 (TIMP-2) inhibits membrane activation of matrix metalloproteinase-2 (MMP-2)” Matrix Biol. 2011 Sep;30(7-8):404-12. (PMID: 21839835) Robichaud TK, Steffensen B, Fields GB. Exosite interactions impact matrix metalloproteinase collagen specificities. J Biol Chem. 2011 Oct 28;286(43):37535-42 (PMID: 21896477) Poster Abstracts: Robichaud TK, Carruthers. A Mutagenesis of the Human type 1 glucose transporter exit site: A functional study. ACS 234th Meeting, Boston MA. Division of Biological Chemistry, 2007 Robichaud TK, Bhowmick M, Tokmina-Roszyk D, Fields GB “Synthesis and Analysis of MT1-MMP Peptide Inhibitors” Biological Chemistry Division of the Protein Society Meeting, San Diego CA 2010 Robichaud TK; Tokmina-Roszyk D; Steffensen B and Fields GB “Exosite Interactions Determine Matrix Metalloproteinase Specificities” Gordon Research Conference on Matrix Metalloproteinase Biology, Bristol RI 2011

INSTITUTION AND LOCATION DEGREE (if applicable) YEAR(s) FIELD OF STUDY Oakland University, Auburn Hills MI BS 1993-1998 Biochemistry University of Massachusetts Medical School, Worcester MA PhD 2001-2008 Biochemistry & Molecular Pharmacology University of Texas Health Science Center, San Antonio TX Postdoc 2009-Present Biochemistry

Awards and Honors
1998 Honors College Graduate, Oakland University 2009 Institutional National Research Service Award, Pathobiology of Occlusive Vascular Disease T32 HL07446 2011 1st Place, Best Postdoctoral Poster, Dental Science Symposium, UTHSCSA, April 2011

Past/Present Clients
Invited Seminars: Robichaud TK, Fields GB. “Synthesis and Analysis of MTI-MMP Triple Helical Peptide Inhibitors” Pathology Research Conference, University of Texas Health Science Center San Antonio Pathology Department (June 18th, 2010) Robichaud TK & Hill, B “How To Give A Great Scientific Talk” Invited Lecture, Pathobiology of Occlusive Vascular Disease Seminars, UTHSCSA (Nov 11th 2010), Cardiology Seminar Series, Texas Research Park (Feb 21st, 2011) Robichaud TK; Tokmina-Roszyk D; Steffensen B and Fields GB “Exosite Interactions Determine Matrix Metalloproteinase Specificities” Gordon-Keenan Research Seminar “Everything You Wanted to Know About Matrix Metalloproteinases But Were Afraid to Ask” Bristol, RI (Aug 6th, 2011)

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