Because there will be a redundancy and we can't design the primary key for these tupules. Thus we can't uniquely identify the record.

2.Why do we designate one of the candidate keys of a relation to be a primary key?

In order to uniquely identify complete tupules in the relation with one attribute.

3. What is the difference between a key and a superkey?

A superkey can have redundant attributes while a key cannot.

4.Pick a nucleotide sequence of your interest using specialized or integrated databases.   Find in the databases by keywords.  Save the sequence to a file and publish on your own webpage.      Underline the accession number, organism, author(s),  medline number, start and stop codon on nucleotide sequence.    Do the link to related sequence(s) in other databases.

LOCUS       HIVU50628     287 bp    mRNA            VRL       04-APR-1996
DEFINITION  Human immunodeficiency virus type 1 envelope gp120 V3 region (env)
            mRNA, partial cds.
ACCESSION   U50628
VERSION     U50628.1  GI:1255167
KEYWORDS    .
SOURCE      Human immunodeficiency virus type 1.
  ORGANISM  Human immunodeficiency virus type 1
            Viruses; Retroid viruses; Retroviridae; Lentivirus; Primate
            lentivirus group.
REFERENCE   1  (bases 1 to 287)
  AUTHORS   Sova,P., van Ranst,M., Gupta,P., Balachandran,R., Chao,W.,
            Itescu,S., McKinley,G. and Volsky,D.J.
  TITLE     Conservation of an intact human immunodeficiency virus type 1 vif
            gene in vitro and in vivo
  JOURNAL   J. Virol. 69 (4), 2557-2564 (1995)
  MEDLINE   95191036
REFERENCE   2  (bases 1 to 287)
  AUTHORS   Sova,P.
  TITLE     Direct Submission
  JOURNAL   Submitted (05-MAR-1996) Pavel Sova, Molecular Virology Lab, St.
            Luke's-Rosevelt Hospital Center, 432 W 58th St., New York, NY
            10019, USA
FEATURES             Location/Qualifiers
     source          1..287
                     /organism="Human immunodeficiency virus type 1"
                     /proviral
                     /specific_host="patient 8 PBL of Multicenter AIDS Cohort
                     Study"
                     /db_xref="taxon:11676"
                     /map="7070-7363, according to HIV-1 HXB2 strain"
                     /clone="8.18"
                     /note="PCR-amplified and cloned proviral DNA fragment from
                     short-term PBL coculture"
     gene            1..287
                     /gene="env"
     CDS             <1..>287
                     /gene="env"
                     /codon_start=2
                     /product="envelope gp120 V3 region"
                     /protein_id="AAA96047.1"
                     /db_xref="GI:1255168"
                     /translation="IIRSKNFSDNTKTIIVQLKESVKISCIRPNNNTRKGIYIGPGRA
                     IYATGKIIGDIRQAHCILSKEKWNDALKQIVTKLREQFPNKTIVFNQSSGG"
BASE COUNT      133 a     39 c     49 g     66 t
ORIGIN      
        1 aataattaga tctaaaaatt tcagtgacaa tactaaaacc ataatagtac agctgaagga
       61 atctgtaaaa atcagttgta taagacccaa caacaataca agaaaaggta tatatatagg
      121 accagggaga gcaatttatg caacaggaaa aataatagga gatataagac aagcacattg
      181 tatccttagt aaagaaaaat ggaatgacgc tttaaagcag atagttacaa aattaagaga
      241 acaatttcca aataaaacaa tagtctttaa tcaatcctca ggagggg