Editing Francis Crick (section)

==Directed panspermia==
During the 1960s, Crick became concerned with the origins of the genetic code. In 1966, Crick took the place of [[Leslie Orgel]] at a meeting where Orgel was to talk about the [[origin of life]]. Crick speculated about possible stages by which an initially simple code with a few amino acid types might have evolved into the more complex code used by existing [[organism]]s.<ref>{{Cite journal|author=Crick FH |title=The origin of the genetic code |journal=Journal of Molecular Biology |volume=38 |issue=3 |pages=367–79 |date=December 1968 |pmid=4887876 |doi=10.1016/0022-2836(68)90392-6|s2cid=4144681 }}</ref> At that time, [[protein]]s were thought to be the only kind of [[enzyme]], and [[ribozyme]]s had not yet been identified. Many molecular biologists were puzzled by the problem of the origin of a protein replicating system that is as complex as that which exists in organisms currently inhabiting Earth. In the early 1970s, Crick and Orgel further speculated about the possibility that the production of living systems from [[molecule]]s may have been a very rare event in the [[universe]], but once it had developed it could be spread by intelligent life forms using [[Spaceflight|space travel]] technology, a process they called "[[directed panspermia]]".<ref>{{cite journal|url=https://profiles.nlm.nih.gov/SC/B/C/C/P/_/scbccp.pdf |archive-url=https://web.archive.org/web/20050912214232/http://profiles.nlm.nih.gov/SC/B/C/C/P/_/scbccp.pdf |archive-date=2005-09-12 |url-status=live |title=Directed Panspermia|author1=Crick, Francis |author2=Orgel, Leslie E |journal=[[Icarus (journal)|Icarus]]|year=1973|volume =19 |pages =341–346|doi=10.1016/0019-1035(73)90110-3|issue=3|bibcode = 1973Icar...19..341C }} Crick later wrote a book about directed panspermia: {{Cite book |author=Crick, Francis |title=Life itself: its origin and nature |publisher=Simon and Schuster |location=New York |date=1981 |isbn=0-671-25562-2 |url=https://archive.org/details/lifeitselfitsori00cric_0 }}</ref> In a retrospective article,<ref name="CrickRetro">{{Cite journal|vauthors=Orgel LE, Crick FH |title=Anticipating an RNA world. Some past speculations on the origin of life: where are they today? |journal=The FASEB Journal |volume=7 |issue=1 |pages=238–9 |year=1993 |pmid=7678564 |doi=10.1096/fasebj.7.1.7678564 |doi-access=free |s2cid=11314345 }}</ref> Crick and Orgel noted that they had been unduly pessimistic about the chances of [[abiogenesis]] on Earth when they had assumed that some kind of self-replicating protein system was the molecular origin of life.

In 1976, Crick addressed the origin of protein synthesis in a paper with [[Sydney Brenner]], [[Aaron Klug]], and George Pieczenik.<ref>{{Cite journal|vauthors=Crick FH, Brenner S, Klug A, Pieczenik G |title=A speculation on the origin of protein synthesis |journal=Origins of Life |volume=7 |issue=4 |pages=389–97 |date=December 1976 |pmid=1023138 |doi=10.1007/BF00927934|bibcode = 1976OrLi....7..389C |s2cid=42319222 }}</ref> In this paper, they speculate that code constraints on nucleotide sequences allow protein synthesis without the need for a [[ribosome]]. It, however, requires a five base binding between the mRNA and tRNA with a flip of the anti-codon creating a triplet coding, even though it is a five-base physical interaction. [[Thomas H. Jukes]] pointed out that the code constraints on the mRNA sequence required for this translation mechanism is still preserved.<ref>{{Cite journal
| doi = 10.1016/0006-291X(72)90031-9
| last1 = Jukes | first1 = T. H.
| last2 = Holmquist | first2 = R.
| title = Evolution of transfer RNA molecules as a repetitive process
| journal = Biochemical and Biophysical Research Communications
| volume = 49
| issue = 1
| pages = 212–216
| year = 1972
| pmid = 4562163
}}</ref>