 In this video, I will cover the following objective, describe the structure and function of a eukaryotic chromosome, what is a chromosome, how many do you have, what are homologist chromosomes, what are sister chromatids, and define what a gene is. Here we see a karyogram made from photos of chromosomes in a human cell. So humans are eukaryotes, or eukaryotic organisms, meaning that we have cells containing organelles, and our DNA is found on linear chromosomes, instead of circular chromosomes that are found in prokaryotic organisms like bacteria. Humans contain 23 pairs of chromosomes in each of our somatic cells. The somatic cells are the majority of the cells in the human body. In contrast, the gametes are the sperm and ovum. The gametes only contain 23 chromosomes total. They do not have pairs. A cell that does not have pairs of the chromosomes is known as a haploid cell. It only has one copy of each chromosome. So the gametes are haploid cells. In contrast, our somatic cells are diploid, meaning each cell has two sets of chromosomes. 23 pairs, or 46 chromosomes total, are found in human diploid cells, which are the somatic cells in humans. Chromosomes are made of condensed chromatin. Chromatin is normally uncondensed, loosely spread out inside of the nucleus, and only during mitosis and meiosis, only during the division of the nucleus, will we have the chromatin condensing to form the visible linear chromosomes as are shown here in this photo. You can see the chromosomes are numbered 1 through 22 here. One is the longest pair of chromosomes, and 21 and 22 are the shortest pairs of chromosomes. Although 21 turns out to be slightly shorter than 22, that wasn't originally appreciated when the chromosomes were numbered, and we will keep with that convention. The 23rd pair are the sex chromosomes. Our matching pairs of chromosomes are known as homologous chromosomes, and we have 22 autosomal homologous pairs. So the autosomal chromosomes are the ones that are numbered 1 through 22, and then here we can see there are two X chromosomes in this karyogram. Those are known as sex chromosomes, and because this person has two X chromosomes, we know that their sex is female. In contrast, a male would have one Y chromosome and one X chromosome. Here we can see two homologous chromosomes that are also each already copied. An identical molecule of DNA produced in DNA replication is known as the sister chromatid. After DNA replication, each chromosome contains two sister chromatids, and those chromatids would initially have an identical DNA sequence. The centromere is the region where the sister chromatids are held together, and the kinetochore is a structure within the centromere. This kinetochore region of the centromere will attach to microtubules during mitosis in order to direct the movement of chromosomes as the nucleus divides. A gene is a functional segment of DNA that provides information necessary to build a protein. The central dogma of molecular biology is the idea that genetic instructions move from DNA to messenger RNA in the process of transcription. Transcription is the synthesis of RNA from a DNA template. The genetic instructions that have been transferred into mRNA then exit the nucleus and are read by ribosomes in the process known as translation. Translation produces a protein. A sequence of DNA known as a gene can code for a messenger RNA that codes for a protein. This flow of genetic instructions from DNA to mRNA to protein is essentially the central dogma of molecular biology. DNA is used as the blueprint to direct the production of proteins. Each three nucleotides functions as a codon. Three nucleotide sequence of DNA will be converted to a three nucleotide sequence in the messenger RNA. And then during translation those codons, the three nucleotide sequences in the messenger RNA will be interpreted by the ribosome in order to direct the sequence of amino acids that are incorporated into the polypeptide being synthesized.