After the Earth developed a magnetic field and before single-celled photosynthetic organisms appeared on Earth, what was the atmosphere like? * 1 point
mostly hydrogen and helium
85% water vapor, 10% carbon dioxide, 5% nitrogen
78% nitrogen, 21% oxygen, 0.9% argon, 0.1% all other gases
there was no atmosphere before single-celled orgamisms

/the trachea, commonly known as the windpipe

fertilization, pictured in figure 24.23a is the process in which gametes (an egg and sperm) fuse to form a zygote. the egg and sperm each contain one set of chromosomes. to ensure that the offspring has only one complete diploid set of chromosomes, only one sperm must fuse with one egg. in mammals, the egg is protected by a layer of extracellular matrix consisting mainly of glycoproteins called the zona pellucida. when a sperm binds to the zona pellucida, a series of biochemical events, called the acrosomal reactions, take place. in placental mammals, the acrosome contains digestive enzymes that initiate the degradation of the glycoprotein matrix protecting the egg and allowing the sperm plasma membrane to fuse with the egg plasma membrane, as illustrated in figure 24.23b. the fusion of these two membranes creates an opening through which the sperm nucleus is transferred into the ovum. the nuclear membranes of the egg and sperm break down and the two haploid genomes condense to form a diploid genome.

explanation:

this protocol provides a detailed procedure for co-immunoprecipitation (co-ip) to identify protein–protein interactions.