The commission may adopt the plan as a whole by a single resolution, or, as the work of making the whole city plan progresses, may from time to time adopt a part or parts thereof, any part to correspond generally with one or more of the functional subdivisions of the subject matter of the plan. Before the adoption, amendment or extension of the plan or portion thereof the commission shall hold at least one public hearing thereon. Fifteen days’ notice of the time and place of such hearing shall be published in at least one newspaper having general circulation within the municipality. The hearing may be adjourned from time to time. The adoption of the plan requires a majority vote of the full membership of the planning commission. The resolution shall refer expressly to the maps, descriptive matter and other matters intended by the commission to form the whole or part of the plan and the action taken shall be recorded on the adopted plan or part thereof by the identifying signature of the secretary of the commission and filed in the office of the commission, identified properly by file number, and a copy of the plan or part thereof shall be certified to the council and the municipal clerk, and a copy shall be available in the office of the county recorder of deeds and shall be available at the municipal clerk’s office for public inspection during normal office hours.