A2 Human Biology

The cycle starts when the pituitary gland (at the base of the brain) secretes a hormone called FSH (Follicle Stimulating Hormone) and LH (Leutenising Hormone). FSH enables follicle growth and as it develops it releases oestrogen, which thickens the endometrium.

The oestrogen inhibits FSH and LH until about day 11 when oestrogen concentrations are very high. This high level of oestrogen causes LH and FSH to be released in a surge (POSITIVE FEEDBACK). This LH surge brings about ovulation. The mature follicle bursts and the oocyte is released.

LH stimulates the ruptured follicle to develop into a corpus luteum which continues to secrete oestrogen as well as progesterone which inhibits FSH and LH as well as stimulating the growth of blood vessels in the endometrium. As LH is now inhibited the corpus luteum begins to degenerate and produces less progesterone. This fall means FSH is no longer inhibited and the whole cycle renews.

The fall in progesterone and oestrogen leads to menstruation, in which the outer layer of the endometrium is shed.

Towards the end of pregnancy the concentrations of progesterone (which inhibits uterine contractions) in the blood falls, while the concentration of  oestrogen (which stimulates uterine contractions) rises.

Oestrogen also makes the myometrium (muscles of the uterus) more sensitive to oxytosin, secreted by the pituirary gland, which also stimulates myometrium contractions. Another hormone relaxin is produced to soften the cervix and make the pelvis more flexible, making birth easier.

The pressure of the fetus's head against the cervis stimulates stretch receptors which send impulses to the hypothalamus, causing more oxytosin secretion which creates stronger contraction. An example of positive feedback.

 During pregnancy progesterone and oestrogen stimulate the growth of milk-producing tissue in the breasts-mammary glands.

The high concentrations of progesterone during pregnancy inhibit the production of prolactin but as pregancy finishes, progesterone falls meaning prolactin is no longer inhibited and the production of milk is stimulated.

The first milk formed is called colostrum, and is high in antibodies which give some passive immunity to the newborn baby. The sucking on the teet stimulates the secretion of oxytosin which causes the muscle in the walls of the mulk ducts to contract, squeezing milk out of the teet. It also causes more prolactin to be stimulated, producing more milk.

Prolactin also inhibits FSH and LH production so it is unlikely that ovulation will occur during full-time breastfeeding.

BLOOD GROUP             ANTIGENS ON RBC                ANTIBODIES IN BLOOD PLASMA             POSSIBLE GENOTYPES

          A                              A                                          Anti-B                                  IAIA    or    IAIO

          B                              B                                          Anti-A                                  IBIB    or    IBIO

         AB                        A and B                                   Neither                                         IAIB    

          O                         Neither                             Anti-A and Anti-B                                IoIo

ABO blood groups are inherited and determined by a single gene with three different alleles. This is an example of inheritance involving multiple alleles. IA and IB are codominant, while Io is recessive.

A gene is a length of DNA that codes for one polypeptide, and alleles are different forms of a gene. This means changes in DNA that makes one gene will result in new alleles.

When DNA replicates there's a risk of mistakes, however almost all mistakes are corrected by DNA polymerase enzymes which copy DNA. Uncorrected errors are called mutations.

Some mutations will have no effect as they occur in 'junk' DNA, as a large proportion of our DNA does not appear to code for polypeptides. However it may have a function we do not yet understand.

TYPES OF MUTATION

Deletion- when one base pair is deleted, this will mean that the triplet code will be read wrong for the entire DNA, we say a deletion mutation is a frame shift mutation, because all the triplets are changed after the mutation.

Substitution- when a mistake is made in replication in the one base, however this only affects a single triplet so only one amino acid in the protein will be altered. This is known as a point mutation.

 However one change is still a big difference- e.g. sickle cell aenemia occurs from just one change to an amino acid.